I've been using a couple of them. I picked up one of the energizer cases (very similar to the mophie) for when i know i will be burning through the battery and won't be near a charger.

http://www.energizerpowerpacks.com/us/ap1201/

I also have a Zagg Sparc for the other times. I keep this charged up in my laptop case for the inevitable flight delays where i burn through the iPhone or iPad power. I got this when it was on its 50% off sale so you can keep watching for that if they run it again. I don't know if i'd pay $100 for it.

http://www.zagg.com/accessories/zaggsparq.php