I think the Grabber ones are the ones I generally get on sale at Walmart, but, at various times I've used a few other kinds, and they all seem to work ok when new.

Funny thing is, I seldom use them for what they are intended. <img src="/images/graemlins/crazy.gif" alt="" /> The most common use I've found is putting one next to my Nalgene water bottles, then wrapping the bottle and heater in something like a spare fleece in my pack. It keeps the water from freezing when you're hiking/camping in the winter. One other thing where they are really handy is to periodically rewarm your hands in a pocket when you're doing some fine work where you really need to use exposed hands/fingers.