A couple of winters ago, I did my own little evaluation of said bags.
I took an "Extreme" US GI bag, available for about $25, a "moderate" US GI bag (about the same money), one of the realtively newer 3-piece systems Uncle Sam issues ($300 if you buy it new), and my own idea: a cheap bag, much like a guy can get at any department (like China-mart) with another similar bag put inside. I did manage to find the waterproof bag for the 3 -piece set-up for a fraction of the cost.
On each night I tested, it was about 20 degrees. I rated them against each other, judging merely on how good I slept through the entire night (providing I slept the entire night!).
With each one, I slept on a GI cot, with a thick quilted moving blanket folded up underneath me to keep the bottom warm. I wore cotton "long-johns", top and bottom.
The Extreme was perfect. I was warm, but like any GI bag, not very roomy inside.
The Moderate wasn't quite as warm, but not enough difference to bitch about.
The 3-piece was as warm as the Extreme. A little roomier.
The two China-mart bags, one inside the other, was as warm as the Extreme. A noticable amount of better leg-room.
I did try one of the civilian bags, but about an hour into the test, I soon realized that particular test sample, failed....I was freezing!
Now, I did not test the waterproofing, as it simply didn't snow.
I concluded if a guy had a couple of civilian sleeping bags, and a way to keep dry, in my opinion, I wouldn't hesitate to use the civilian bags.
I recently got a "Merlin" bag. It wads up into a ball about the size of one of those old high school dodge-balls. Warm as can be. Stupid money though.
My $.02.
