Philosophical considerations aside, I can only say good things about these footbeds by PolarWrap/ToastyFeet. Using an aerogel--developed for NASA by Colorado-based Aspen Aerogels--as an insulator, these lightweight, soft-feeling footbeds are comfortable and entirely effective. Over a month spent on Alaska's north shore last February, I wore them inside a variety of boots, including Sorel Bear (rated to -40F) and Sorel Glacier (rated to -100F). Without the footbeds, each of these boots did relatively well, with a simple three-layer sock system: a polypro liner, a plastic bag as a vapor barrier liner, and a heavy wool sock. But still, even the -100F-rated Glacier left my toes a little chilly on -40F days...until I inserted the footbeds. With the footbeds in, there was no issue. None. No cold, even at -40F. I mean nothing. I sometimes keept the boot liners in my sleeping bag, with the footbeds in the boot shells outside overnight; other times, I kept the footbeds in the liner boot, in my sleeping bag. No appreciable difference. In either case, no cold. It's done. The manufacturer's website (http://www.toastyfeet.com/) indicates that the insulation is also effective against hot substrates, e.g. to prevent feet burning while trekking in deserts, and that makes sense. The aerogel insulating material is mostly air, and will insulate against either heat or cold. It's simply a barrier. Although it's mostly air, the footbed can't be deflated by puncture.

After about 70 miles of dragging my sled, the footbeds began to crack a bit, but I patched them up easily enough. Still, they're maybe a little less durable than one would like, but I'm happy to buy a new pair every season. At less than $20US per pair, they're a modern miracle. No cold. I mean none. I can't recommend them highly enough for anyone venturing outdoors into snow. Winter mountaineers, I think, will get a lot of use from these, as will anyone going outdoors in deep cold.