I love my Mophie Helium case. At 1500 mAh, it won't quite give you a full charge (use of power + charging results in less than 1500 being in the phone battery when the Mophie is empty) but it is very thin and light, and honestly, looks pretty good considering. I also have a Mophie Powerstation Mini, which is 2500 mAh. I don't always carry them both with me, but they come in handy for Field Exercises, camping, hiking, Geocaching, etc, when I know I will either be using a very large amount of power, or when I will spend time away from power. As others have said, make sure you buy from a legit source if you do get a power case. If you end up with a fake TPU case, its still most likely a TPU case and not really a big deal, but if you get a knock off Power case, you could end up with a fried phone.
As a side note, if you do get a battery case, to get the most out of it, leave the case turned off until your phone is close to empty, then turn on the case, this results in much better total life from the total power you have on hand. Basicly, treat it like you unplug the phone, then when it is low, you plug it in, rather than taking the approach that it is just one big battery.