My 3GS is over three years old, battery was completely shot, it would die between 100% and 80 - 85% life. Also the power button seemed sunken in and weary, decided to spend the extra five bucks on new power button as well.
Printed the repair guide for battery replacement and power button replacement, followed everything step by step and the battery installation and power button replacement went perfectly. i even feel like my iphone is more well put together than before i reassembled it.
if you think you are one of those people that can diy, go for it its pretty exciting and fun if youre a nerd like me haha. make sure you dont lose any tiny little screws. sucks trying to find them from off the floor.