I've used a Thermarest in my hammock most often, and it does work well. The downside is that the pad and bag slide around separately- I've almost lost the pad over the side of the hammock. Still, after a little bit of learning curve the combo works down to cold temps.
I've got a friend who uses his Big Agnes bag & pad (not sure which model, but the bag has the integrated pad sleeve) in his ENO Doublenest and loves the combination.
Bookmarks