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.