I love this thing. I bought it on a whim at a local store, and I have found so many uses for it, it's ridiculous. I used it as its own outer layer in mid fall and spring temps, an amazingly warm inner layer for skiing (the two layers trap a surprising amount of air), outside layer for rock-climbing...just about anything I dish out to it. Surprisingly warm as an inner layer, and very windproof as an outer layer.
I am 5'9" and weigh 165 lbs and a medium is a great fit...highly recommended