My iPhone 3G would not hold a charge longer than an hour or two, so I clearly needed a new battery.
No problems. It took about 20 minutes and although the battery was a bit difficult to remove from the adhesive, all went well.
Read all the steps of the repair manual before starting. I thought the video was the manual and got in a little over my head before realizing that there was a more detailed text version. On my table I had a piece of masking tape, sticky side up, to which I attached all of the screws.