Assuming it's not terrible quality and won't explode on you, any of these kinds of power packs is safe to use for your phone. Phones can't be "overcharged"--they'll draw only as much current as they need.
5 volts or 5Mah? Most chargers are 5v but vary from (.500 to 2.1a) Ive used a iTaste MVP electronic cigarette battery before to charge my phone. It has a usb port on the bottom for that purpose. It throws about 8 volts but the amps are at. 2.1 which was fine with my phone. Charged