"This thing has saved me so many times"4.0 starson by albania8989
Pros: Extra battery without external dongle-like devices. I never lose the juice pack 'cause it's always on the phone. It wraps around the phone, so it gives some added protection.
Cons: I have to carry a micro USB cable with me to charge it, it doesn't use the Apple connector or a mini USB connector.
Summary: If you need extra battery this is really the way to go. The added protection is a plus.