Same here my battery life is already good. But I'm gonna go ahead and reccomend this app to you and everyone else. I've been using my phone pretty constantly for about three hours, I'm at 90% battery and juice defender is quoting 130% battery life compared to normal. I've heard people say they get well over 200% battery life and ill update this tonight before I go to bed with final results.
Edit: ok my final number was x1.68 batery life. I suppose it did last a lot longer than usual. After seventeen hours with moderate/high use I'm still at 40%. However it does seem this app is increasingly effective the less you use your phone, because the main way it saves power is cutting services while the screen is off.