I've bought two from Amazon, admittedly less expensive ones. In my experience, they use very low grade Li-Ion batteries that don't hold a charge well. The result is that when you need them, they're dead unless you're constantly topping them off. We already have too many rechargeable batteries to keep track of in life, so I don't use them any more.

If you DO buy one, get a name brand one, as they're more likely to use a decent quality battery. They're making them quite small now...