Got one for christmas and am writing this review as I try it my first time. I have had it on for about 15 minutes now, and am happy with this gift.
While the other review is correct in saying that it doesn't emit a lot of heat, it does get about as warm as you can expect from something powered by a USB. Warm enough to make a nice difference on these cold winter nights. It just might be cheaper to get a pair of thick fluffy slippers, unless you spend a LOT of time sitting, as I do.