We're testing the '06 model (with the zippered footbox) right now and will be publishing a full review in the Spring.
I own an earlier model. Its double layer loft is approximately 2" (1" single layer). The insulation is very evenly distributed due to their welded construction.
Sorry, but I don't want to spill too many other observations about the bag right now with a full review pending. I will say that based on field experience, loft measurements, and bag construction, I think their 40 degree rating is overly optimistic.