I am a stomach sleeper and like to spread out, and so my Montbell Super Spiral 30 degree down bag is the most comfortable mummy type bag I have ever slept in. I was always uncomfortable in the military in the ?10 lb Military 2 mummy bag/1 bivy sack sleep system when it got below 45 degrees and had to actually zip it up. The only thing possibly more comfortable for me so far for winter camping is using two big square 5 lbs each car camping bags with one cinched around my neck tightly and the other loosely covering my head to create a warming chamber to breathe in.
I have my bag in a Montbell sleeping bag cover and use a Sea-to-Summit liner so I don't ever have to wash the bag, condensation off the Tipi can't get the bag wet or if I want to bivy with just a poncho shelter, and to protect the bag's very thin/lightweight materials. This whole set-up weighs 37 ounces and I have slept down to 12-15 degrees comfortably with my fleece hat on on a couple of occasions. My wife who is a much colder sleeper than me has the Super Spiral Burrow Synthetic 30 degree bag with Sea-to-Summit silk liner and has slept comfortably at 15 degrees with her base layers on.