I like my Mophie because i can leave it standby mode and it's not constantly charging my iPhone. I bought it because so far i'm not happy with battery life on the 4s and nothing is worse than going out and not being able to use your phone late at night . That's when you want to capture all the stupid stuff you see in NYC lol.
I bought one of the Energizer cases too, it's a good deal and i can give it as a present for Xmas to one of my iPhone friends.