My iPhone 3gs started acting crazy, it would shut down randomly, even if it was 100% charged, except when it was connected to the power.
Since I consider myself "The geek of the family" I decided to risk $20 bucks plus my iPhone and decided to do it by myself. I ended up saving $30 bucks if I had taken it to a cellphone repair place, and I learned more about iPhone.
Everything went smooth until I got to the battery, it grave me some troubles to take it off, but finally everything went perfectly.
Always have the installing directions on hand.
Print out or draw a layout of your device to place the screws where the belong to, so that they will not mix out. DON'T make lever from the top of your iPhone/pod when taking the battery out, mine made a crack sound, I thought it was broken but everything turned out great.