I use the Mophie Juice Pack Plus because it doubles my battery life and provides a speaker box that enhances the speaker on the iPhone. I'm very happy with it and it was recommended to me by someone on this forum.
I used to use a Griffin Survivor and it was huge, heavy, and ugly... This case protects my phone just as well and has a MUCH slimmer profile. I also now use a shield for the front of the phone.
Bob