Most of the well know brands make a solid bag. It helps if you understand the fabrics and fill types. Some bags have a much better DWR and not all down works as well in damp. How much this is going to matter to you will depend on what conditions you camp in. I have found that for extended camping in the cold where tents often get covered in frozen condensation it gets harder to keep bags dry, they soak up moisture from the tent and absorb it from the sleeper. A 0F bag won't be a 0F bag on day 10. If your sleeping in a heated tent it won't matter near as much. As folks have said the pad under you is important. Lastly look at the sizing/Girth of the bags. Its easy to get drawn to a bag that is warm, but light to find out that the bag is cut quite narrow. For me I would want a bag that is large enough to be comfortable even when wearing all my layers.
All of this^^^
Used a WM Antelope GTX bag for years. Problem with it was in the September archey monsoon season when it was humid, just cold enough to rain/sleet/snow all at the same time, is body vapor would eventually "wet out" the down. Plus mine was the first version of GoreTex and even "breathable" shells slow down the transfer of vapor from a sleeping bag. The antelope was a great bag for backcountry skiing during the winter but I switched to a synthetic bag for archery hunting where temps generally don't get much below 25 degrees.
The advent of water resistant down has allegedly changed that according to some friends who work in the outdoor clothing industry. So I bought a KUIU 0 degree Long bag last fall when they were on sale. The bag has just enough girth to fit me well even with my base layer and a light polar fleece jacket on ( I'm 6-1, 195-200lbs). I spent two nights in it elk hunting with my son and it rained--in November at 9400ft no less (stupid global warming) and there was some condensation in the tent. I couldn't detect any dampness in the bag, even when I got home and pulled it out of the stuff sack.
Since then I have spent one night backcountry skiing when the temps outside the tent were 12 degrees in the morning. Of course inside the tent was a lot warmer. Based on those three nights I'm liking the bag.
Casey
