Anonymouswrote a review of Big Agnes Zirkel SL Sleeping Bag: 20 Degree Down on April 24, 2009
Pros: lightweight, high quality down and fabric material, width and size is roomy and comfortable for a mummy bag.
Cons: Temperature rating of 20F is very optimistic. Using a hammock or tarp as a shelter, and wearing thermal pants and wool baselayer with hat, I felt slightly cold at temp of 35F with 10-20 mph winds. Perhaps without wind and inside a tent would be good up to 25F. I think I would feel very uncomfortable below that. In addition, if you inflate the pad all the way, you will have a real difficult time zipping the bag up past the shoulder level.
You can probably get a lighter and warmer bag but it wont be as roomy as the BA. So for now BA bags are a good compromise.