I bought this for a member of my family, as the medication she's on leaves her hands feeling like little blocks of ice in the cold weather - she's very happy with the relief it brings to her. While suffering from Raynaud's, my sister also decided to give it a try, as nothing else she's tried in the past has been any help, and this big flat pebble manages to give her a bit of a boost now & then.
With it's 3 settings (Off/Med/Max) I found the medium setting to be adequate, as the max setting was slightly too warm for comfort in my case. The battery seems to have a decent lifespan between charges (when on medium setting) and is charged via a USB port or wall socket using the included cable & 3-pin adaptor. The device takes around 3 hours to charge from flat & while heating up very quickly, it only heats up on one side. We only use the included soft-cloth drawstring pouch for storage, as we find the handwarmer to be less warm otherwise.
For anyone in reasonable health with hands that feel the cold now & then, I would say this is a useful little gadget. But for those with health problems which precipitate really cold hands, then this may or may not bring some relief to your condition.