I used one overnight once and don't recall any condensation. I believe it's all about proper ventilation.
I'm going to do a real test with my Ridgerunner and Spindrift. I'm hearing reports of a 15 to 20 degree difference between inside and out. With that much difference I'm hoping I can get away with my WM 30 degree bag down to 20 or lower. That would be pretty sweet especially since my zero degree bag is synthetic and heavier than heck.