This jacket is the best jacket I've ever spent my money on. Don't know if I would have paid full price initially (got it on steepandcheap.com) but now, after owning it a year, I definitely would.
Regarding sizing - I'm 5'5" 125 lbs and I got the medium. Fits very well, loose enough that I can layer a t-shirt, and an Outdoor Research fleece jacket (also size medium) underneath, and be comfortable down to about 30 degrees F (O degrees C), even if I am just standing around, 10 degrees if I am hiking or something. It's a snugger fit with the fleece underneath, but still very comfortable.
On the other hand, I've also worn it on a 60 degree day, when the breeze might be a little chilly, opened up the pit vents, and been perfectly comfortable.
The wind-stopping nature of this jacket makes it a great one for cycling as well!