It's a ittle sad to see Mophie not making its usual hard cases cum battery packs for larger phones, because unless you want to put a brick sized device in your pocket, everything will look ugly. Mophie has built its backup battery pack for both a USB and MicroUSB cable into its Juice Pack Reserve Micro. It's good because now you can charge any device that's not coming from Apple.
The $40 price tag is reasonable for the fact that it'll give you at least half a charge from zero, At least. Look for larger models that will give you a bigger charge instead.