I just purchased an Incipio Battery Case for my 4S. I'm actually trying to compare it to the Mophie Air. Can anyone her give me a quick review of theirs - pros and cons. I noticed inside the box of the Incipio Battery Case includes 2 rechargeable batteries. Does one of these batteries equal in the same amount of usage (time) as the Mophie Air, which has a built-in battery into the case? Incipio is claiming 3x the battery of the 4S, whereas Mophie Air provides an additional 75-80%. Thanks.