I dont own this but my hiking buddy does. He is a cold sleeper and even at a windy low single digits he said he was plenty warm, no extra layers needed. Draft tube and collar gave him no grief.

I researched many different bags before buying this one. I needed a bag for winter/snow camping for the Central Oregon Cascades. This one met all the criteria in needed; down insulation for warmth and loft retention, draft collar, suprisingly ample room and ease of movement, and sufficent water reppeling ability. But as with any down bag, laying out in the rain or snow is not a good idea. Tent or Bivy is always recommended. Marmot hit a home run with this bag. For any winter camping, this will always accompany me. Loft retention is great, lots of room (especially in the foot box), all-around great bag for cold camping.

I have used this bag in temps ranging from 5 degrees F to 50 degrees F (more as a blanket at that point).

Let me first say that the bag is warm and quite comfortable and if there were no design flaws or customer service issues I would highly recommend the bag.

At the very lowest temperatures I have used the bag in, it has kept me warm other than a very annoying problem with the draft tube designed to insulate against cold air coming through the zipper. This design gets an F from me because every time you move, air will come through. There is no mechanism for it to stay put, such as small tabs of velcro or a windproof flap on the outside.
Although the fabric that the bag is made of does retain down, the seams DO NOT! Initially, the amount of down lost was minute...maybe 5-10 feathers per trip, but after about two years of use and maybe 15 nights in the bag it has been losing more like 25 feathers or so through all of the seams and stitching.
Final complaint - the draft collar that you cinch around your neck has become unstitched from the rest of the bag and there is now a two inch space where down flies out of. One night with this hole and it almost looked like snow inside my tent. When I spoke to the Marmot rep about their willingness to resolve the problem, the response was to pay for shipping to them, they would sew it up and send it back. Not sure how others may feel about this but first, why did this problem happen on a $400 bag and second, why would Marmot not be willing and quite honestly anxious to take care of the problem quickly and quietly? Very disappointed in the customer service received.

Excellent hood, extra-lofty, light and conservatively rated. The Couloir is a classic mountaineering bag, built to last. Down is far more durable than synthetic insulation, so you'll likely be warm in this bag for years to come. And the price isn't bad either!

I have owned this bag for a little over a year. I recently used it on a Rainier climb and it performed flawlessly. It is super lightweight, has nice loft and breaths extremely well. I would recommend it for any winter outdoor activity.

This bag is amazing! I am more comfortable and get better sleep when I'm in this bag than when I'm sleeping in my bed. Really warm and has a silky-smooth liner. A lot of bags say they are 0 degree but are actually significantly colder after they are used a few times. Not with this bag. The 800+ fill is super warm and does not pack out. Another bonus is the roominess inside the bag; plenty of wiggle room. On a cold, clear night there is no need for a tent when you have this bag. The only catch is, you do not want to use this bag in wet conditions w/o a bivy or a good tent. Absolutely no complaints with this sleeping bag.

I took this bag out on one beast of an inaugural run. I don't really get the name, but I got the fact that it kept me toasty warm with nothing but a waterproof cover and a thin foam pad. Just that to protect me froma thick blanket of snow and the gusty breeze atop a mountain in central Utah near the mouth of Kolob Canyon, in December of 2008. Now I know that Central Utah mountains aren't usually as cold as the Wasatch Range, but this was right after a huge snow storm rolled through and unusually cold weather (dropped to around -5 at one point). I'm not sure what our altitude was, but I think that this trip spoke volumes for the Coulair. I didn't even have a true bivy, just a rain cover to stop any melt from getting in, and I slept pretty much naked in this beast. I actually broke out into a light sweat the first night. So the next night I wore my smart wool thermals and they helped me regulate my temperature a little better and I slept pretty well. I would recommend this bag to my friends. It packed to about the size of an oversized football (cylindrically spoeaking)l, the smallest out of anyone in my group and arguably performed the best. I just hope this bag is as durable as I hope over time. I give it my seal of approval.

I have owned more than my share of sleeping bags in my life, and this is my favorite. The loft is great, the pack-down size and weight are good and the temperature rating is pretty good for me, and I tend to get cold at night. I love this bag.