I had several pads and then ended up selling several, save the Xtherm. 13-ish oz for nearly a 6 r value rating is tough to beat. I succumbed to temptation this past Spring and picked up an Uberlite (also from Thermarest) for summer trips, 8-ish oz and have had it down to freezing a couple of times w/o any problems.
If temps are expected around 0 or below, I add a ccf pad to the Xtherm.
