We didn't feel like we should have to pay $$$ for an Apple "Genius" to diagnose the obvious, that my phone battery needed to be replaced. It wouldn't hold a charge and the phone would shut off after about 5 minutes of use, even though I'd charged it all night.
Hubby and I did ourselves for under $30 and felt great being able to do it without taking it to Apple. You need a good uninterrupted half hour in which to do it, but it'll probably take you less time than that. Simple if you follow the instructions carefully because there are lots of little parts.
Make sure you are working on a clean surface and that you keep all the little screws and parts organized (away from pets and their hair/fur!).