Because some of Amazon's listing descriptions for Icebreaker items have very few details about the products, making it difficult for the customer to make an informed purchasing decision if he/she is unfamiliar with this brand of clothing, below I'll type some information and personal impressions about Icebreaker clothing.

Note that my review is quite long and usually only the first half of it is shown on this page. If this happens on your screen and you wish to read the second half, all you have to do is press the "read more" button at the bottom of the review and Amazon will show you the rest of it.  :-)

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OVERALL IMPRESSION

Icebreaker's marino wool items are generally made well, last a long time, are easy to care for, feel fantastic on the skin, are "natural", don't get smelly easily like polarfleece does, let the skin breathe, etc.

Many of my Icebreaker items have been worn and washed several hundred times and are still holding up much better than would be expected.

I am a BIG fan of this company's products.  For my overall experience of them from around 2000-2010, I would give them a solid "5 stars"!

However, I regret to say that my opinion of the items I have bought from Icebreaker in 2012 is not as high.  In the last 2 weeks (late 2012), I have bought 7 items from them and I decided to return all of them because they aren't as well-cut as the very "same" items that I bought in the 2000s (which I still have and wear, so I can make like-for-like comparisons easily).

Based solely on my 2012 experience of Icebreaker products, I would give them 3 stars, "it's ok".  For me, this is not good enough to command prices of $50 - $100+ for quite simple items, even if the basic wool material itself is outstanding and long-lasting.

For my star-rating on this review, I will average the "5" I give Icebreaker historically and the "3" that I would give Icebreaker in 2012, and give them a "4".

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MY EXPERIENCE WITH ICEBREAKER

I bought my first Icebreaker clothing about 13 years ago when I needed some long underwear for skiing.

Up until about 6 years ago, I had been buying Icebreaker items very sparingly, due to the eye-watering prices at London UK sporting goods shops .  Then, I discovered that Sierratradingpost sells various Icebreaker styles (usually discontinued items from years past) on their site for a discount. Fortunately, for a couple of years after I discovered this, they had a large selection of women's Icebreaker stuff, so I bought quite a few things for great prices. (These days, Sierra's Icebreaker selection for women appears to be smaller, and the prices comparatively higher, than before.  I expect that there are more discounters they have to compete with now, to get that kind of stock.)

In addition to bricks-and-mortar sports shops and Sierratradingpost, I have also bought Icebreaker items from several sporting goods online retailers, Amazon, and the Icebreaker website.

I still have almost every Icebreaker item I've bought.  Even the discounted items have been somewhat pricey, but the quality and longevity were worth the initial investment.

In the last two weeks, I have tried to replace a few of my key Icebreaker items but found the fit and cut of the current versions of the same items to be sub-par.  I will keep using my old items for another year, and hope that next year's fit and cut will be better-suited to my body shape.

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CAVEAT

What I have written below is based on my current knowledge about their products, but it is possible that I have misunderstood some aspects of the brand.

If you wish to better understand their product offerings, I would absolutely recommend that you have a look at the Icebreaker official site to view their entire range and gain more clarity about what items are for what purposes, what the wool weights are about, what the quality levels of the different clothing collections are, etc.

(Unfortunately, their website, while snazzy, does not fully explain these things, leaving it up to the customer to piece some of the puzzle together.)

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ICEBREAKER WOOL WEIGHTS:

Icebreaker's marino wool items generally come in four weights of fabric: 150, 200, 260, or 320. This is "grams per square meter".

150 is quite thin.
It provides light warmth.
The material is generally see-through.  (I wouldn't wear items of this weight out and about in public without anything over them, but some reviewers have said that they do.)
It's good for a first base layer, for strenuous exercise, or for sleeping in.
Their items of this weight, especially in the ranges intended for athletic use, tend to be really form-fitting in one's typical size.
All Icebreaker material seems VERY resilient and long-lasting to me, but because the 150 wool weight is pretty thin, it is the likeliest to be torn, develop holes, get "runs" (like pantyhose).  I don't think this happens too much with it, but I've seen some reviewers mention it (most often talking about how they accidentally poked holes in it with long fingernails), and three of my 150-weight leggings have developed runs in them after several years of heavy use.

200 is slightly thicker, but still on the thin side.
It's pretty warm.
The lighter colors are definitely see-through, such as grey and cream. My older black and brown items made out of 200 weight wool don't seem to be see-through, but some of the newer dark-colored ones might be.
It's good for a base layer or mid-weight layer, for exercise, and sleeping in.
Especially in the ranges intended for athletic situations, their items of this weight tend to be form-fitting in one's typical size.

260 is on the thicker side.
It's definitely warm, a cozy weight.
None of my 260 items are see-through. I can wear tops made out of this weight on their own (without anything over them) and not worry that my underwear or too many lumps&bumps will show through.
It's good for a mid-layer or top-layer, and sleeping in (if you get cold at night, like I do).
Items of this weight tend to fit looser than items of the same size in the 150/200 weights do. This may have something to do with how much naturally-stretchier the thicker fabric is, but it also seems that they cut these 260-weight items bigger because this weight is often worn as a top-layer or middle-layer, instead of as a base-layer.
I like this weight the best.  I think that the company says that this is their most popular weight across the board.

320 is thickest, of course.
It is quite warm, very toasty and sturdy.
This weight is generally a top-layer, so items made out of this material are relatively loosely-cut.

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ICEBREAKER LINES / RANGES / "COLLECTIONS" OF CLOTHING:

Icebreaker makes different ranges - some intended for wearing as normal underwear (like panties and sports bras and tank tops), some intended for wearing as thermal/long underwear, some intended for wearing as everyday clothing, and some intended for wearing as athletic wear.

Even though at first glance most of their items appear VERY similar to each other, there are different feelings to the fabrics (even in fabrics of the same wool weight), different quality levels, different cuts, and different design details amongst these different lines, so it is good to check their website about the particular Icebreaker item you are thinking of buying to make sure that it is intended for the activity you want it for.

Two of the popular ranges at the lower end of the Icebreaker price scale are:

Bodyfit
The Bodyfit range seems to be for athletic situations, but also casual-wear and sleeping-in. The knit of this range's wool is usually flatter and less stretchy than the knit of the ranges that are for underwear or purely casual wear. Items in this range are usually cut to fit relatively snugly.

Everyday
This range is made of a looser, thicker, stretchier knit than the Bodyfit, and is more for general sleeping-in and casual long-underwear needs, not so much for athletic situations. (However, I'm sure that they would be okay in many athletic situations, and I have worn them skiing and hiking.) Everyday items tend to be made of a stretchier knit and cut to fit more loosely than the Bodyfit items. I wear a size below my ordinary size in this range.

Other ranges
Their other ranges appear to include: Black Sheep Collection, RealFleece Collection, Pure Plus Collection, Journeys Travel Collection, Premium Underwear Collection, GT Technical Sports Collection, GT Bike Collection, Technical Base Layers Collection, and Technical Mid Layers Collection. Please see the Icebreaker site for more information on their ranges.

Frankly, I feel that they've made this aspect of their brand over-complicated and too confusing for the average customer, and I have not been able to locate any overview of the various collections on their site or on their product packaging materials.

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ICEBREAKER COLORS:

Icebreaker tends to make most items in black, as their basic universal color.

Many items are offered in about 2 to 8 other colors, as well.  There is often some version of blue, red, grey, and green - sometimes a pink, sometimes a brown or purple, etc. Often the colors are skewed towards the cool end of the spectrum (which I notice because my coloring is on the warm end of the spectrum and cool tones look awful on me).

The different weights of wool and/or different ranges (e.g., Bodyfit, Everyday, Journeys Travel Collection, etc.) will tend to have their own mini-set of color options.  (For example, the current grey color in the 200 Bodyfit is a medium-grey and is called "blizzard" and the current grey color in the 260 Bodyfit is a dark-grey and called "metro", and they don't offer those particular colors in other weights of wool or other ranges, as far as I know.)

They change their colors every year or so (I think), and some colors tend to sell out mid-way through a season and are not restocked, so if you see an item in a color that you love, or there is a matching set (of long underwear or something) and you want both pieces in the same color, you probably should get it at the time, because it's not guaranteed that it will be available in the future.

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EXTRA DESIGN FLOURISHES:

A couple of their ranges, mainly the more athletic lines, feature "pops" of contrasting color in a certain part of the garment, such as a stripe running down the sleeves or down the sides, or a triangle of bright material at the ankle. Most internet retailers' photographs of the garments do show these features because the areas of differing colors are pretty obvious, but it pays to read the item descriptions (and check out the manufacturer's site for confirmation) if this kind of thing is not what you are looking for.

In a couple of their clothing ranges, mainly the underwear, 150-weight, and casual-wear lines, some of the tops and bottoms have printed patterns (like floral motifs or abstract swirls - sometimes in contrasting colors) on the sleeves or in the hip-region. This *can* escape your notice when the only images of the item you have available to you are the tiny product photographs that most internet retailers provide, so do double-check the customer reviews of the item, and maybe the Icebreaker site's own full description of the item, if you want to know for sure if the item comes with a printed pattern on a section of it.

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ICEBREAKER SIZING:

Sizing and fit are so variable from person to person and from brand to brand, but in case it may help, I'll talk about my experiences with Icebreaker clothing.

For reference, generally I am usually a women's size 4 or 6 in US sizing, a women's size 8 or 10 in UK sizing, and a small when it comes to small/medium/large sizing.  This goes for both tops and bottoms.

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Sizing of Tops

Almost all of my Icebreaker tops from the last 13 years -- crew tops, long-underwear tops, turtlenecks, and zip-up jackets -- are women's size small.

The only tops I have in size extra-small are from the Skin range (which they phased out in 2006, but the sizing and type of material used for the Skin range appear to be very similar to those of the current range called Everyday).

Generally, I know that an Icebreaker small top is probably going to fit me.

The main issue that I've had with their tops is sleeve length (too long).  This is not uncommon for me, because I do have arms that are on the petite side:  most suit jackets I buy have to be shortened a little in the sleeves, and I often have to turn up sweaters one turn at the cuff.  And one feature of some Icebreaker tops is a thumb loop which necessitates that the sleeves are meant to be about three inches longer than the wrist - I understand this, and just roll those tops up a turn or two.  However, I've read quite a few reviewers' comments that mention that Icebreaker sleeves tend to be long, and I do think that their sleeves are longer than normal.  ["Normal" being subjective, of course!  ...the only women I've met from New Zealand actually did happen to be tall, lithe, and willowy, so maybe that's normal there.  :-) ]  Last week, I bought a 2012-made Icebreaker "Oasis" top in a size small, and while it was too tight in the chest area (which is normally NOT a problem for me with Icebreaker), the sleeves were a crazy four or five inches past my wrist, even though they didn't have thumb holes and therefore were meant to end at the wrist. I had to return it.

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Sizing of Leggings:

I have run into inconsistency in Icebreaker legging sizes. I have some in extra-small, some in small, and some in medium.  (And in one 2012 collection, the extra-smalls are a little too big for me but there isn't a smaller size to go to.)

I have not found Icebreaker's official sizing chart (available on their site) to be terribly accurate for leggings.

For the leggings, it seems to me that one must experiment in each Icebreaker clothing range and wool weight to discover the best size and fit for oneself.

I will spell out below the ones that I have tried.  Because the sizing of the "same" leggings sometimes has varied across the years, I will mention what year I got them.

1.  BODYFIT range

1a. Bodyfit 150 leggings

2006 or 2007 Bodyfit 150 leggings
Size medium. The label says "slim fit" (possibly back then they had different "fits" within the Bodyfit range, but they don't seem to now).
Fit me snugly, but well.
Waistband comes up to the belly button, feels secure.

2008 Bodyfit 150 leggings
Size small.
Are a bit too small for me this winter - they fit me when I'm about 5-to-8 pounds less than I am at the moment.
Waistband hits 1 inch below the belly button, feels okay.

2012 Bodyfit 150 leggings
I decided not to buy new 150-weight leggings in 2012 because after a few years of heavy use, my old Bodyfit 150 leggings developed some un-mendable "runs"/"ladders". I still wear them and love them, but they are slowly self-destructing. It is comparatively easy to snag them on a rough surface or to poke a fingernail through them when pulling them on or off.

Actually, I've just looked on the Icebreaker site (December 2012) and they don't list ANY Bodyfit 150 leggings.  The only 150 weight leggings they list are the "Sprite" leggings from the Premium Underwear Collection.  Also, I think that the "Nature" leggings (which are a discontinued style) were of 150 weight, and I have seen these still on sale on a few websites.

1b. Bodyfit 200 leggings

Current suggested retail price of around eighty (discounted price of around sixty).

2008 Bodyfit 200 leggings
Size medium.
Fit me, and there is extra room in them - I probably could go up 5 to 8 pounds and they would still fit. When I purchased those mediums, I tried on the small size and found it to be way too small, especially along the tight and narrow waistband that these leggings were made with at the time.  (I think it was a reaction to the consumer response to that waistband that they made the newer waistbands really wide and soft.)
Waistband comes up to the belly button, feels secure.

2012 Bodyfit 200 leggings
At first, based on the Icebreaker official size chart, I ordered a medium in these.  They were too big, so I exchanged them for a size small.
The small fit relatively well - snug, but acceptably so.
Waistband came up to 2 or even 3 inches below the belly button. This is a more "modern" position for a waistband, of course, compared to coming up to the true waist, but I prefer "athletic" base layers to cover my entire body - neck to ankle, to enable the base layer shirts to be securely tucked inside the base layer pants, and to feel really secure when I'm moving around -- but this waistband was so low on me, and so wide and stretchy that it felt like it was about to slide down to my hips at any moment because the elastic wasn't very tight.
Not only from the practical side was this low waistband a problem, but MAN, the whole cut of the upper part of these leggings seemed to be designed to make my tummy area look as flabby and unattractive as possible!  I tried my old leggings on when I took these off -- just to check that it wasn't bad lighting or post-Thanksgiving excess weight that was to blame for how these made me look, but my older Icebreaker Bodyfit 200 leggings looked a lot better on me than these leggings did.
I had to return them.  I have decided to wait until next year's production, to see if they alter the cut of the waist area.

1c. Bodyfit 260 leggings

Current suggested retail price of around ninety (discounted price around seventy-five).

2012 Bodyfit 260 leggings
Size small.
Fit relatively well - snug, but acceptably so.
Waistband came up to 2 inches below the belly button. They had the same issues as the 2012 Bodyfit 200 leggings did - waistband too low, too loose, too wide, and quite unattractive in the belly region!
I also had to return these.  I hope next year's will be cut higher on the waist.

2.  EVERYDAY range

2a. Skin 200 leggings (discontinued 2006) - the Skin range was probably similar to Everyday.

2005 Skin 200 leggings
Size extra-small, yet are the largest I own - I could gain 15 pounds and they would still feel okay. But they fit me nicely when I'm at any weight - 10 pounds less or more than my present size - they are simply the coziest and softest that I own.
Waistband comes to about 1 inch higher than belly button, feels secure.

2b. Everyday 200 leggings

Current suggested retail price of around sixty (discounted price of around forty-five).

2012 Everyday 200 leggings
Two weeks ago, I bought them in size small which were too big, so I exchanged them for an extra-small.  Even the extra-smalls had extra material in places on the legs, which is unusual because my legs are not toothpicks (this kind of legging is not supposed to be baggy, so the cut of these must have been off).
Worse, the upper part of the extra-small leggings was cut strangely - the waistband was in a deep V, so that the middle was 3 inches below my belly-button (which is way too low for me), but the sides rose up 3 inches higher than that (it wasn't a curve, it was a visible V).  Yet the waistband elastic wasn't tight enough, so the tall sides wanted to sag down and bunch up around my hips -- strange.  (And that was not the way the smalls I tried on the week before had fit my waist area!  In fact, I think the smalls of last week fit me better generally than the extra-smalls of this week.)  I don't have the energy to go through the rigamarole of ordering a different pair of extra-smalls, but maybe I just got a bad apple.
I had to return the extra-smalls.
I was very disappointed because they were the 7th Icebreaker product I've tried in the last 2 weeks.  This Icebreaker shopping quest has taken up a fair bit of my time lately, but I have nothing new to show for it.  I've given up trying to get anything now and will wait a year and see how their 2013 production fits me.

3.  OTHER ranges

Their other legging types are more expensive than the Everyday and Bodyfit, and seem to be geared towards high-tech athletic use, or constructed to be worn on their own as pants/trousers. Some have Lycra in them. Check out Icebreaker's site for more info.

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OTHER TIPS

The other main retailers of this kind of product sometimes offer free shipping on their Amazon listings of an item but don't offer free shipping on the exact same item when sold through their own site (even when the price of the item itself is the same on both sites), so it can save you money if you pay attention to that.

The best selection BY FAR is available from the manufacturer's site, but the prices there are generally higher. However, they have discounts from time to time, such as their "Black Friday" sale when they offer 30% all items that are black in color.

Amazon sometimes calls the Icebreaker Bodyfit range "BF" and sometimes "Bodyfit", so if you are searching here for Icebreaker's Bodyfit range, do a separate search under each of the two search terms to get the most accurate results.

In terms of merino wool socks, I prefer Smartwool socks over Icebreaker's. Sierratradingpost is one retailer which often carries a large selection of them at good prices. Those socks last a really long time. I wear them almost year-round because the lightweight ones breathe really well even when it's warm out.
The Smartwool women's small size is the only sock I can find that truly fits my feet (which aren't even that small, all things considered!) without bunching up inside my shoes.
If you have a smaller foot (size 7 or less) and are female, note that the Smartwool unisex socks in the small size are bigger than the smalls made just for women, even though that company's size guide says that they both are meant to fit the same range of women's shoe sizes.