I've been using Aspen aerogel blankets to make multilayer insulation for instruments and vessels at the lab where I work for a couple of years, and I aquired a large piece of Aspen's Spaceloft and experimented with sleeping pad designs about eight months ago. I reported all of the details and carried on some discussion in a dedicated thread:
Aspen's aerogel blankets are very dense compared to typical sleeping pad foam. I tried using the lowest density product Aspen makes, the Spaceloft 6250. It's only 6mm thick but close to 20 ounces/square yard. A torso pad would be half a pound before encapsulation. The fibrous mat in which the aerogel dendrites are formed is like heavy industrial felt. They can be peeled apart to give thinner sheets, but this causes a good deal of the entrapped aerogel to be lost as clouds of fine, choking dust. Flexing the blanket also produces a lot of dust. Any place that folds soon loses all it's aerogel and becomes just a polyester mat.
Also, a lot of research has gone into the use of aerogel-based multilayer insulation for underwater pipelines, where compression of the insulation is a problem (as in sleeping pad design). The aerogel is a bit like that brittle green foam often used for artificial plants. Under compression it mashes down irreversibly. It's not elastic. In the studies I read of pipeline insulation the compressed aerogel doubled in density and lost half its initial insulation value.
I think an aerogel sleeping pad competitive with plain old EVA foam in performance would need three things: a shiny IR reflective envelope (aluminized mylar, for example), evacuation (to below 10 torr), and incompressible posts (maybe syntactic silicone foam) to protect the aerogel from compression.
Ultimately you'd get a very expensive, fragile, complex system that probably wouldn't beat EVA by very much.
It wouldn't suprise me a bit if one of our little MYOG community came up with a way around these difficulties, though. I don't want to shoot anyone down. I think powdered aerogel (pulverized granules), vacuum sealed in tiny (1/8 inch) tubelike parallel baffles, might have promise for warm, lightweight, waterproof gloves and hats.