I have used this bag on Denali several times. It has kept me warm down to 30 below. Eating a half stick of butter in your hot chocolate before bed helps. Also, when awoken from cold, having a candy bar on hand to munch down is always a good idea and lets me get back to sleep. The zippers are strong and the bag has incredible loft. It cinches around my head very well. I take off my jackets for sleeping and have long underwear and fleece with my puffy jacket over my feet for added warmth. I have used this bag in the Tetons in winter when it has been severely cold outside and I was toasty warm and smiling. It is pretty heavy, but that is the price for warmth! Have a great adventure!

I bought this bag for dog mushing trips in interior Alaska, where the temps really let you test out a bag's rating. This one does the trick! Of course, you do need to wear long underwear, hat, and warm socks, but you should anyway. It's just one more thing you can do to be nice to your expensive equipment.

This bag lofts like no other, it'll look like it's going to take over your tent when you pull it out of the stuff sack. The water resistant shell helps you out if you spill your coffee, but will work against you on longer trips. Perspiration has a little harder time working it's way out, so for this reason I recommend a vapor barrier liner for any trips more than maybe a week in length. I like Western Mountaineering's liner, but you'll have to find it somewhere else, as Backcountry.com doesn't have it, unfortunately. I also recommend getting a waterproof compression sack for it, I've been using Granite Gear's Drylite Rock Solid in a size Large. I really like it, it works much better than the stuff sack supplied with the sleeping bag. Find it here at:
http://www.backcountry.com/store/GRG0004/Granite-Gear-Drylite-Rock-Solid-Compression-Sack.html

I bought a long, even though I'm only 5'9". I like the extra room so I can put booties on my feet for when it's really cold, or for throwing my damp clothes at the bottom. Also, be prepared to fight this mammoth a little bit when stuffing. Not only is it huge, that shell that keeps your coffee out of the down also likes to limit how much air moves through it. So as you squish it down, it will tend to poof up like a balloon. You just have to keep at it, keep squishing. I've found that I'm able to stuff mine in about 2-4 minutes.

This thing is a down cave!!! If the temp is just arround 10-15 degrees, you might sweat to death...
I would feel safe sleeping in this thing at the northern most tip of alaska without a tent in the dead of winter!!!