The sleeping bag I use year round is "rated" and I say that with a cough and rolling eyes, but, anyhow, it is "rated" at -30F. All I have to do to be comfortable in it on a 50* night is unzip it. I have to wear serious long johns to bed to be comfortable at -10F in it. For that bag, which I actually like, a lot, the "rating" is absolutely, 100%, utterly, completely meaningless.
All that said, I spent a few nights in it this past weekend in temps between about -15F and 10F. Never felt the least bit cold or uncomfortable. Slept wonderfully well on the -10F night, in fact.
It's the one that weighs about 15 lbs. One of those oversized - like 40" x 84" or so, canvas covered, flannel lined jobs. I've had it about 15-16 years now. Spent, oh... I don't know, conservatively, with no chance of exaggeration, maybe 450 nights in it? Probably a lot more than that, as I've actually kept track only the past two years and have about 100 nights in it just in 2012 and 2013. No complaints...
- DAA