This will be the warmest lightest layer in your arsenal. Under a hardshell and over a base layer I have been comfertable down to 15, and with a few more layers I would take it down much colder. It has saved me from hypothermia several times in the mountains. Definitely get the hooded jacket... it is much warmer! They fit tight to make them more thermally efficient, but it is not uncomfertable. If you like a larger fit then get a size larger than you normally wear.

I wore this jacket nearly every day on a recent trip to Europe in late Novemeber.
It was incredibly warm and worked well with my Marmot Sharp Point to keep me warm as I walked and stood around cities in the low 40s with only blue jeans, a t-shirt and the UL Parka and my SP.
I threw it in my bag during the day and when the sunset I had a great layer to throw on. Its ability to be stuffed to a small sack was great for all-day outings when the temperature would drop 20 degrees over the day and the wind would pick up by night fall.
My only qualm with it is that it is constantly loosing feathers. When warn underneath my SP, the SP would have feather sticking to it. Even now, you can see feathers coming through the a few of the thread holes. I'm not sure about its durability, but it was handsdown the warmest thing I brought. This with a fleece and a waterproof Precip was more than enough to keep my upperbody warm while standing around in temperatures in the 20s.

Last week up in the Sierra's at 11,000 feet, lows in the high teens or low 20's, with a base and a light fleece under this jacket as the 3rd layer, I was toasty and with its' ultra light weight, free to move.

Outrageous value.

A very very very light parka, I got it for winter camping so light is good, however, these parka's are (I believe) made in China and are not American sized,they seem to run at least 1 size smaller than US made. I got a medium that fits like a glove, barely enough room under it for a base and second layer. I am 5'8" 140lbs. Regular 37" chest. No zips on the pockets let in blowing snow.

I traded up from a Mountain Hardwear Sub Zero and am happy with the change. The MontBell is lighter, packs into a very small stuff sack (included), and I like the hood. Only thing I would like to see changed is a zippered pocket somewhere. As far as fit goes, it seems to fit pretty true-to-size for me...I have the large and am 6 ft., 170 lbs. and it fits just right.

Great parka, lightweight, compactable, warm and good price quality ratio