Tried cheaper battery - wound up buying ifixit
20 - 30 Minuten
Replaced my 3gs iphone battery - but found it cheaper on amazon - I thought. Bought and inserted the new $7 battery, but my phone went berzerk. Found out there are only certain battery runs which are compatible with 3gs - other groups don't ensure you'll get the right one - ifixt battery (and superior tool set) was $15 and is now in and running my phone without a hitch.
Easy peasy, 15 minutes - great online manual, reading the reviews helped give a few tips to make it even easier - fantastic experience.
The ifixit screw driver is already magnetized - tape a piece of masking tape to your work surface sticky side up to hold the little screws as you take them out, or use a refrigerator magnet to keep them in place while you work. There are two large ones for the case, and the rest are the same size and interchangeable, so don't sweat exactly where they came from.