Rating: 4 / 5
There are not that many bags you can use in really cold weather, but this is one of them.
If you're into really cold camping this is a great bag, weighs about 5 lbs, cost $550 - 650, it will also make you sweat above 20 degrees F. With 800 / 900 fill, it packs down small for the expanded size. The foot box is a nice size, and for such a warm bag it's not too difficult to turn over in this bag.
If you need a compact (relative to the task at hand) single bag for -20 to 0 weather, this is a good choice to consider.