8:42 p.m. on November 28, 2012 (EST)
I have a NeoAir XTherm. I've used it about 6 nights now, all in different places. I am a very cold sleeper, so I didn't expect it to live up to it's "truly 4-season" claim, for me at least.
In above freezing temps, it was, as expected, warm.
On top of snow, I expected to be way too cold, but surprisingly, I was only a bit cool. It was well below freezing. I didn't measure, but my boots were frozen in the vestibule and my water bottle was slushy in my pack (which was in my tent inches from my head). When I put my hand under myself (on top of the pad on top of the snow) and left it there, it was actually somewhat warm.
I may have slept cool because my extra quilt (over my bag) kept slipping off. I will have to sleep on top of snow a few more times, at temps when my water bottle freezes harder, to really know, but so far, much warmer than I thought it would be. I would say for a "normally" warm sleeper it would be good.
On top of earth, but with snow all around, and similar temps as above, I did feel warmer, but again, it could have been the bag arrangement.
Someone warned me that the crinkling sound might bother me, but even though I am a light sleeper, it didn't. I haven't noticed baffles breaking inside, but how would I know?
What was annoying was how long it took to inflate it with the stuff sack pump, but I didn't want to breathe in it and add water into it every time I used it. But I suppose any pad with a large volume like the NeoAir would take time to inflate.
All in all, so far I am surprised at how well it performs, not to mention that I no longer have to dig a little "butt depression" under the tent to be comfortable side sleeping. I've never BP-ed with a pad this thick, and it sure is nice. And light.