This may not give any really relevant information to Jim, since he wants to go down to zero, but I just spent 2 nights sleeping on the 66" NeoAir All-Season pad in high-teen to low-twenties weather in Virginia, on ground that had been cold for a while. I didn't actually sleep on the snow or exposed rock, but there was still snow on the ground in a lot of spots. I was in a full-net Carbon Reflex tent, with the fly buttoned up; the wind blew hard both nights, and a little of it made it into the tent.

The NeoAir really impressed me. I never got cold, in the sense that you could feel the cold seeping up through the pad. (I share Jim's low opinion of the BA Insulated Air Core pad; I felt myself getting cold at 35 degrees on it one night.) In fact, quite the opposite: you could actually feel your body heat being reflected back up to you. One other person in our party was using the same pad in the 72" length, and he reported the same results.

When you first lie down on the pad, or when you shift positions so your legs or feet are on a spot that wasn't previously covered, the pad feels cold; but in less than 30 seconds, you feel the heating effect start. (It's that same kind of warm "glow" you get when you first put on a wool sweater or down jacket - I've never gotten that "glow" with fleece.)

So, to at least 15 degrees, I'll endorse the NeoAir All-Season.