My iPhone 3G battery would not hold a charge and I can't stand the thought of spending a bunch of money for something simple.
The video and the step-by-step instructions were very clear. Of course, the screws are incredibly tiny and my fingers are large, so it was sometimes hard to line things up. The whole thing took about 30 minutes and it worked perfectly. The advice provided about expected issues like prying up tabs and removing the screen was right on the money.
You should lay your phone on something like a smooth cloth so the screws don't get lost. I used a light colored placemat. I also placed each screw on the placemat near where I took it out because they are not all the same. That way, they all went back together the right way. Also, buy the screwdriver, spudger and suction cup. The spudger and screwdriver that come with the battery are too small and the better screwdriver you can buy also has a spinning handle that makes putting the screws in very easy.