The battery on my old iPhone 3G (which I had to use after my 4 got stolen) barely held an hour of charge.
The repair went smoothly (this was actually the 3rd time I've replaced a 3G battery with a replacement from iFixit, but the first time for myself). As usual, the iFixit guide was EXTREMELY helpful, full of the small details that come up during a fix.
Two things: First—it wasn't exactly clear that the iPhone Replacement battery included a screwdriver and a small plastic spudger. I would recommend just using the included screwdriver with the kit (it is magnetized, unlike the other screwdriver sold by ifixit, for easily removing and replacing screws), however, the plastic spudger included breaks really easily and is not much help at all. Buy the extra spudger and the small suction cup. Second—put some double sided tape on a sheet of paper and mark up a quick guide as to where the screws came from. Some of the screws in the iPhone are marginally different sizes, and aside from the fact that it's easy to loose them (the tape helps keep them in one place), marking up the paper will help you keep track of where they came from.
iPhone 3G Replacement Battery
Phillips #00 Screwdriver