I saw one the other day and was thinking it might be a good addition to a backcountry kit. Not sure if they actually would work in the extreme conditions in which you would actually want the damn thing (preventing frostbite?)... but it's only 10 bucks so maybe I'll give it a go.
I think a friend of mine had one of these for a short while, but it was either generally underwhelming / difficult to use, or it broke. Either way (from my hazy recollection) I wouldn't recommend - surely there are lighter options for keeping your hands warm?
My mom has one that was my grandpa's and I've used it a couple of times when hiking and xc skiing. Best thing about it is you have a lit fire in a bag that you can put in your pocket but then you have a fire in your pocket and that is a little unsettling because the thing is not totally sealed up. It also works as a lighter. I will probably play with it again sometime but I wouldn't take it along for anything too crazy.
A buddy of mine had one and from what I remember, he said that it did keep his hands warm to an extent, but didn't do the job he was looking for. If its only $10 just get it and see if it actually works