Not sure why these haven't seemed to catch on in North America but I am sure glad I have one. Just read much of the surviving a cold wet night thread and can't believe that this little peice of kit wasn't mentioned. Basically a vented sil-nylon bag with floor pullouts you take it out of it's integral bag and cover yourself with it. Mine can seat 4 and is great in winter or summer. Weighs just over a pound. Packs the size of a one liter bottle and unlike a lot of NA equipment the stuff sack is oversize and not hard to repack.
This fellow's writings convinced me and after use I think he is correct.
http://www.ukclimbing.com/gear/review.php?id=1163Anyone else use one of these?