Doug recommends them on the gear advice; linked to from this sites home page.
From what I've read you can get into one with wet clothes and wake up the next morning with the clothes and bag dry.
They are also the only bags that can be compressed without performance loss. The Alaskan national guard actually pack them shrink wrapped in their survival kits. You'll lose 60% of the insulation in 6 months if you do that to a down bag.
qjs