Sizing was interesting. I think the perfect size would have been Medium and 1/2. If I were just getting one to wear out to the local pub and look the part, I'd have gone with the medium. I went with the large so that I could accommodate a base layer and midweight fleece under it, and still have good shoulder mobility for scampering over rocks and ice. With the draw cord around the waist too, I figured a little extra air would keep me that much warmer. And if I fall off a mountain I want to be able to freely flail my arms as I yell "I regret nothinggggg..."
I haven't been out in the backcountry in it yet, but wearing it on my errands today in 7 degrees with 40+ mph gusts, I can't believe how warm it is. And I love its packability. So much more compact than my fleece jacket folded up like a blanket.