Just got my first night on the Neoair XTherm. It got down to about 40, on a clear, windless night in Ohio; I was in a tent with the rain fly on, using a 40 degree hoodless down sleeping bag, wearing lightweight long john top and bottom.
It did just fine; I could feel the heat reflecting back up to me. I was using the 48" version, and it seemed a bit narrower to me, but I think that may be the severe rounding at the top creating a perception rather than a fact. It took about an hour, but I completely adapted to it and never had any problem staying on it.
It appears to be fairly durable - at least as durable as the original and all-season NeoAirs I've had. I never had any problem with leakage, and I used all of them in chair kits for sitting to cook supper.
I found it to be quite comfortable. If you get one, start with it fully inflated, then lie on it. It will probably feel "hard" or "bouncy." While lying on it, open the valve (it will be up by your right shoulder), and let the air out slowly until it feels just right.
I think the reason the reviews are all over the place is that they haven't been around long enough for anyone to develop any thorough experience. This means that people are writing the reviews about comfort before they've really learned how to tweak it in use. I'm not sure they've been around long enough for any real, solid evidence about long-term durability; many of the leaks and other durability issues may be due to user error (I remember similar arguments about the original Thermarests leaking; they all went away as people learned how to use them.)
So, will you be throwing money away if you get one. Personally, I side with Lori: I've never had any problems with NeoAirs, and don't anticipate any. (Yes, you have to take a little care with them - you can't beat them up the way you can closed cell foam. But that little extra care is worth it for the far greater comfort.)