"I've used mine a couple of times to provide quick warmth to a scout who's sleeping bag isn't keeping him warm enough."

In my tent-camping days I used it for the same thing. I'd wake up freezing at 3:00 AM, push the button, and I'd be good to go until it was time to get up <img src="/images/graemlins/smile.gif" alt="" />. Mine lasts a lot longer than 15 minutes though. I'd say I get 2 hrs of useful heat out of it per use. And it is reliable. About half the chemical ones I've used over the years have turned out to be duds.