Just got this jacket earlier in the week and had the chance to test it out last night while watching the Geminid meteor shower for a few hours last night just outside of Denver, CO. Temp was low 30s (felt like mid-20s) and there was a good wind going. With just a thin merino wool t-shirt on under it, I was 100% comfortable temperature-wise, while the friend I was with, who was wearing 4 or 5 layers, was shivering for most of our meteor shower watching.
What I really like about this jacket is that it fits well, with a longer torso and nice long arms. You can easily layer over it or under it, and it doesn't look baggy when it's being used solo. Also, the brim on the hood is great since it doesn't get in the way and can be molded to whatever shape works best for you. The zippered hand pockets are warm and roomy, as is the chest pocket, which doubles as a storage bag for the jacket.
I can't speak to how well the jacket breathes under physical activity, but I can tell you it is definitely warm when you're just sitting around, which is something I was a little nervous about because this jacket is 750 fill down (as opposed to some other similarly priced jackets that are 850 or 900 fill down).
Go by the US size (ex - UK 10 does NOT equal US 10)...and know that this material won't be stretchy at all so, if you're between two sizes, size up.