They're handy as hell, because if your wallet is ever stolen, they can't charge your credit card like a normal one; they can only spend what you've put onto the card.

I had my wallet lifted about 12 years ago while at a club, and they were just starting to come out with these. After getting the credit cards cancelled, and getting the illegal charges settled and proven, I applied to get my CC changed to one of these instead. They still work like a normal CC, and they improve your credit rating through using one, but you don't have the worry of having it stolen and someone charging several grand on it and you being stuck with the problem. Sure, you'll lose the couple hundred you've dropped onto it, but it's less of a problem than the other.