I returned a 150 weight Icebreaker because it is too lightweight for my Fall/Winter northern Nevada walks, I then ordered the 200 weight shirt which is what I need for my use mostly with a soft-shell over it. Fit of both the 150 and 200 weight tees is just as shown in the advertising photos, formfitting so no extra material to fold or tuck, and sleeve length just below my wrist break, perfect for me as the layer closest to my skin. I also like the neckline is not tight or high for a base layer so if I add a scarf or zip up my jacket high I don't feel as if I'm being choked, I don't like turtlenecks with modern high collar outerwear.

Merino wool seems to work for me in cooler/cold weather, and I've also ordered from amazon a pair of Icebreaker merino gloves and two merino beanies. I consider the 150 weight too light for my use, but it might work for warmer temps because it is very breathable, I plan on trying a short sleeve merino tee in 2013.
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Update 10 May 2013
Bought the same model and brand, different size, for my wife and she agrees these are great shirts for a base layer, very warm for the thinness of the material, and not itchy on our skin.  These are skintight and I'd advise considering going up a size if your on the border, in my case I'm a big XL so the XXL I bought works better for me, less stretching of the material especially taking the garment off when it's been worn and damp.

And I have bought a short sleeved version of the same Icebreaker brand in 200 weight, more comfortable for me in some weather conditions than the long sleeve, but I now know it'll be for cold weather use only and I will not be using merino wool in a lighter weight or in warm temps.