I have been using those for months now on my back. At first I was kind of wary. They don't seem to heat up very much. After about fifteen minutes, they are happening. I love those things. Love them. It sucks that they're so damned expensive. For the lower back box of two (they look like cotton weight belts, or, even worse, pants with an expanding wasitline) cost $8.99 out here. If they were cheaper, I'd wear them all day. I would make custom suits out of those magical charcoal like things.