I used this all winter as a baselayer, almost exclusively for alpine skiing. It is soft, not the least bit itchy and I found myself wearing it around the house because it is so damn comfortable. Due to its light weight, it is not that warm on it's own but when paired with the stoic comp 230 anorak and a patagonia nanopuff and a Trew Pow Funk jacket I was toasty in a variety of conditions.
I also use it for cold weather running and it wicks moisture and breathes great. I just wore it under a t shirt for a very short hike at mesa verde and it kept me warm when I needed it and didn't smother me when the sun came out or there was a steep uphill.
As for the sizing, I am currently 5'8.5 155 but when I got this piece I weighed about 170-175. I got a large and it fit alright then but now I would certainly go with a medium. Overall, great piece and I will purchasing more.