I bought one of these heater-thingies, on the assumption that a purpose-engineered device might well be safer and/or more effective than a can of rocks.
The interior glows a bright cherry red at high heat settings on my very ordinary butane stove, and I'd guesstimate that it puts out roughly as much heat as a 1500 watt electric heater. It does indeed seem to be considerably more effective than just cooking something, but because of the low mass the tradeoff is that it doesn't hold heat very long. Overall, however, I'm tentatively a satisfied customer. I'll need more experience before committing firmly.
Two notes...
1) I had to find a very large steel washer to put under this thing, because otherwise it wouldn't sit level on my stove's burner and I felt it was dangerously close to tipping over. This washer is 5" across and weighs maybe a pound. When I use it the heater less glows brightly and seems to put out a bit less heat, but not enough to make a practical difference. My guess is that the heavy steel washer is simply absorbing some of the energy and radiating it instead of the device.
2) I like the fact that the _inside_ gets glowing-hot and not the outside. If it were to somehow be knocked off the burner, it'd be that much less likely to start a fire.
Will this thing make a van/small trailer toasty warm in zero-degree weather? I can't answer that because I live in Florida. But... As I said, I'd estimate it as roughly equivalent to a 1500 watt electric.
Hope someone finds this useful...