I can't speak to that bag specifically, but I have the Big Agnes Mystic, which is also a 15 degree down bag. I love the sleeping bag for comfort because I toss and turn a lot when I sleep, and the integrated pad set-up is great for keeping me from becoming a tangled mess. However, the bag doesn't even come close to 15 degrees. I'm not a very cold sleeper or a very warm sleeper...I'm somewhere in the middle. But even with top and bottom long underwear, this bag gets too cold for me below 32-35 degrees. Below about 33 I have to wear a down layer on top of the long underwear, and below 25 I just give up and use a warmer bag.
I really like the bag for summer trips, but for me it's straight-up a 1.5 season bag in the Rockies.
