I have used this bag on Mt. Elbrus during a winter accent, the weather did get down to -10 for a night at camp 2 and I was pretty comfortable, mind you I had a liner and a bivy sack inside a 4 season tent with an insulated floor with an R value of around 15, but I was able to sleep comfortably. If the weather was any colder, I probably would not have made it for the night I was on the razors edge. I think the most optimal weather for this pack is 20-10 degrees. This bag breaths really well and retains heat, which is the most enduring quality. This bag packs down to about a #3 or #4 soccer ball, so it does not take up a lot of space for a winter bag. With most winter bags it does have a little weight too it, but for winter enthusiasm I think this has been my experience most of the time. This bag is a 600 down fill, and in the future I think I would get a 700-800 fill bag because there are more options out there that are affordable. Bottom line, the bag worked and kept me toasty. This bag is priced right and I would recommend it for anyone doing any moderate winter activities.