The gear that really makes an impression is the stuff that's tough enough to bring into the harshest environment, yet is ridiculously light and packs away small. A perfect example is the newest NeoAir air mattress from Therm-a-Rest, the XTherm, weighing in at 15 ounces.
The warmest pad in the NeoAir line, the XTherm claims an R-value of 5.7 (an indication of warmth; the NeoAir XLite, by contrast, claims an R-value of 3.2). That's a pretty lofty number, and although we can't scientifically verify it we can say that it's a damn warm pad. The XTherm doesn't use insulation per se, instead relying on its 2.5-inch thickness, clever construction and the use of reflective barriers to conserve heat.
In the field, the XTherm proved easy to live with. You can blow it up the old-fashioned way, about 18-20 breaths, or you can use about the same number of pumps from the included combination stuff sack/air pump. It's only an ounce and a half addition to your kit, and it's ingenious: Just slip the hole in the end over the valve, close the end by bunching it up, and push the air into the pad. At altitude, you'll appreciate saving your breath and, even better, you'll avoid introducing moisture from your breath into the pad. Very cool.
The surface of the XTherm is grippy enough to keep you from sliding off, assuming you haven't camped on the side of a serious grade (say, 75% or more). The only wrinkle in the system is just that: the pad crinkles every time you move. If you can live with that, ther's no better pad for cold-weather adventure than the XTherm. It's going up Denali on our next trip, paired with another Therm-a-Rest standout, the Z Lite Sol. It's available in three sizes, and includes a repair kit.
Hits: Packs ridiculously small and light, provides awesome warmth, has a non-slip surface and its stuff sack doubles as inflator.
Misses: Open your wallet wide; people who need durability more than anything should look elsewhere.