My battery at my iPhone 4S was draining to fast even when it was at stand-by so I had to change my battery or have a charger with me everywhere. I picked the first choice.
My phone is still covered by its warranty by Apple but authorized iPhone service places in Greece where we do not have Apple Stores really sucks.
I preferred to fix it myself instead of giving it to those service centers which for me is a great risk for the device.
My order arrived really fast (about one week). Firstly I inspected my new toys, the plastic opening tools and the 54 Bit Driver Kit. Then I started disassembling my phone. Surprisingly the P2 (pentalobe #2) bit from the kit did not fit in my screws but iFixit sent me another bit as soon as I emailed them with my problem.
The replacement bit arrived about one week later and managed to unscrew the screws and open my phone.
Changing the battery was not a difficult procedure and everything went good.
The only part I was afraid is the part where I had to separate the old battery from the phone, a was afraid putting too much pressure not to harm the device.
If you try this do not be afraid as it is very simple just be really careful with the screws, while trying to unscrew them with the defective screwdriver I ruined one of them and had to use a dremel in order to pull the screw out.