The bag's outer layer is packed with 800-fill down that's been treated with a water-resistant chemical called DriDown so the feathers don't clump together if they get damp. In other words, it will keep you plenty warm. But instead of completely sealing you inside with a zipper, the bag's inner layer works more like a duvet, letting you pull it up to keep warm, or kick it off in the middle of the night to cool off. Brilliant.
At $400 for a 15 degrees version, and $350 for a 30 degrees one, it's not considerably expensive, at least compared to other ultralight but ultrawarm sleeping bag options. But for those of us who cringe at the thought of crawling into a sleeping bag, it's worth every penny. More here.