+1. I do this head to toe, including some nice, dry wool socks (or camp booties, even better). The result is an instant toaster effect when I crawl into my sleeping bag. Plus, I have a layer of armour against the 4 a.m. cold should I need to take a short, uh, stroll.

I guess I would try the rock thing on a survival exercise. Probably on the foot end. Seems like a lot of fuss though.