I've been using Marmot down bags for about a decade now: a 15 deg. F Helium, a 40 deg. Arrete, and a minus 20 deg F something or other. All have 800 or 900 fill down and are fairly pricey. But the interesting thing is they seem to be trying to apply some science to the rating system. The hang tags now have a multi-degree rating system, with Comfort, Lower Limit, and Extreme (Survival) temperatures. For instance, the Arrete is a 40deg bag, with a 32deg lower limit and a 20 deg survival, iirc. I used it in New Mexico last month and overnight temps reached 34 or so several nights and I was not "comfortable" but I was able to sleep sporadically. Then, in the mountains it hit 26 overnight and I was close to "not surviving." Seriously cold, the coldest I've ever slept. But I survived. So I guess the rating is pretty accurate. For what it's worth, I had 5 layers on that night. If wearing only one layer, I don't know what would have become of me.

(And please, I don't want to derail this discussion, but with 23 years of light and ultralight backpacking experience I am well aware of the claims of some people that sleeping naked paradoxically keeps one warmer. Suffice it to say that I cannot speak for anyone else's experience, but nothing could be further from the truth with my body. The warmer I am getting into my bag, the warmer I stay throughout the night. I've tried it both ways and everything in between, many times, and that's how it works for me.)

Move cursor over the temp rating box and you'll see a "click to learn more" appear. This explains their system. (This particular bag is the Pinnacle, also another fine bag, just a little heavier and cheaper than the similar rated Helium, which seems to keep me a little warmer.)
http://marmot.com/products/wms_pinnacle