I bought a brand new GI Sleep System for winter use. I used it camping last January in the Boundary Waters. I had a wood stove in the tent but we didn't leave a fire burning overnight for safety, so basically it was warm in the tent for about 15 minutes and then quickly became whatever the ambient temperature was. I noted -5 F to +5 F in the tent on waking up to start a fire.
The GI system kept me warm all night in those conditions. Additionally, I was on a cot with a sleeping pad(not a commercial one, but simply a 6' long roll of Reflectix insulation, it's like space blanket bubble wrap, very light), and wore my merino long underwear in the bag.
The best part of the GI bag was the outermost bag has a reverse hood, that you can put over your face, without it being ON your face. This helped keep my face warm without feeling suffocated, although I generally woke up to icicles from my breath.
I'm positive there are lighter bags, or more compressible bags, but the modularity of the GI bag for the price makes it a great value to me. And although weight/size is less of a concern when trecking overland and ice with a toboggan, some of the big cheaper bags are simply too big and heavy for me personally. The GI weighs about 10lbs and compresses quite well.
The last reason I chose that bag was that if I were forced to hole up overnight without pitching our tent, I felt that the design was better suited for that use than a simple nylon/down bag (even though it might have greater R value).
ETA: I am 6'1" and 280 lbs with broad shoulders. I am the absolute limit on the size that can fit in the GI bag, as zipping it up around my shoulders is a bit of a challenge. Height was fine.