I had two 3g iPhone's that had to be charged every day even when they were not used much.I had replaced one before with a battery that i bought from eBay, witch did good for a few months but suddenly the battery just lost its charge. So I started looking for a battery that came with a guarantee and I bought these from ifixit with a 6 month guarantee.
I bought some Phillips screwdrivers from sears to take the screws out, but was disappointed because they did not last long. The instructions from IFixit were great and saved me a lot of problems. When I put the phone back together the touch screen did not work quite right. Sliding the lock screen slider was real touchy. and at times it would not move if I did not hold my finger just right. also some of the letters on screen would not want to type. I opened up the phone and pushed down on the connectors to make sure that they were seated good, did not notice any that were lose. But using another screwdriver I found that I had several screws that were not tight. The phone seems to work fine now and will stay up for several days.
A good screwdriver would help a lot. I would like to know where to get one that would last for a while. I also found that a magnifying glass on a stand was a big help.