This is my favorite baselayer for cold weather hiking, backpacking, or just hanging around in the cold.  I've tried many kinds of long underwear, from silk to synthetic and nothing beats Smartwool midweight, in my opinion.  They are so versatile, I end up wearing them around the house on chilly days and nights.  I'm reviewing both the top and bottom (long) in this review.  Here is what I have found after wearing them for years:

Pros:

*  Warm - This is essential, as I use these as a base layer for cold weather activities.  They keep me very comfortable and warm in freezing and sub-freezing temperatures during low-to moderate activity.  They make an excellent base layer when combined with appropriate layers of outerwear.

*  Wicks Moisture - These do an excellent job of wicking moisture during moderate to strenuous activity, keeping me dry.

*  Comfortable - These are made from merino wool.  They are extremely soft and non-scratchy.  I am very sensitive to itchy clothing and I can wear these all day, every day, and not have any issues from itchiness.

*  Insulates When Wet - Wool continues to insulate even when wet.  This is especially important if you are in the back country and happen to get caught out in the wet and the cold without rain gear.

*  Odor Control - Something about Smartwool really keeps the odor away.  I've been on cold camping trips where I kept my baselayer on for days and nights on end; my synthetic baselayer ended up reeking early and greatly.  Not so with the Smartwool, under identical conditions.  This is no small thing and it really does make a huge difference.  It must be the wool and I don't know how they do it, but I'm glad they do.

*  Wash and Wear - These do not shrink and do not need special laundering treatment.  I've had sweaters that I've washed before and they came about the size my cat would be comfortable in.  These are machine washable and I can't tell that mine have shrunk even a bit.

*  Durable - These get tough and constant wear when hiking or backpacking and are exposed to strenuous activity.  I've had them for years and they have worn extremely well.  I would rate the material as above average in quality for the use they have received and they still look and perform great.

Cons:

*  Price - Yep, this is a lot to pay for underwear, no doubt.  It is more expensive than synthetic, maybe even double.  From my experience, however, these are more comfortable against the skin, by far, than synthetic gear of equivalent insulation, wick moisture better, and are MUCH less prone to odor with continuous wear.  They are high performance articles of clothing and, though expensive, provide good value for the price.

Other Thoughts

After wearing many other kinds of base layers, these from Smartwool are by far my favorite.  Silk is comfy, but doesn't insulate as well.  Synthetic insulates well, but doesn't feel as good or wear as well as these.  In my opinion, these are the best next-to-skin base layers at any price.  They are super comfy, breathable, and warm.  Also, they look sharp enough that I end up wearing the top by itself on cool days when there is just a slight chill in the air.  Highly recommended.