The Fusion is the lightest 20 degree synthetic bag The North Face makes as well as one of the lightest available. To do this, they took out the full zipper and added a footbox zipper instead, used lightweight materials and zippers, and their lightest filling. I haven't gotten cold in it, even while snow camping.
I actually got the bag as a replacement for my old Cats Meow. I took the bag into TNF to have it cleaned, and they said its lost some loft, would I like a new one (for free) or to upgrade. Since its lighter then the cats meow, and they were only charging $50 for the upgrade, I got it. The reason I mention this though is to point out how good The North Face is. My cats meow was working fine, and the basically gave me a new one, no questions, just because it didn't quite meet its rating. How many companies do you know which will do that? Certainly none in the tech industries, and even very few in the outdoor gear business. Most would at least need to be convinced you need a replacement, not simply offer one!
So its the best bag I've used, perhaps not the lightest, and the lightweight material could lead one to question the durability, but with North Face standing behind it, I have no worries.