my iPhone 3GS was dying with 85-90% charge remaining (lie!!). I didn't want to get stuck with a "new" iPhone 4S + contract, with the iPhone 5 just around the corner.
Taking the phone apart was a little daunting, but the manual was excellent and I got through it just fine. Got the spudger too, which was awesome - I ended up using the little pointy end a fair bit as well. The tool that comes with the battery seems a bit wide for the task, but might work for other folks.
Couple things - First, the "do not remove" sticker is impossible to remove without shredding it. Perhaps the replacement battery should come with a replacement sticker as well. Second, when reassembling the phone, it was hard to know by sight if the connectors were, in fact, reconnected, because my big fingers were in the way. Keep in mind that they sorta "snap" back together, and feel for that little click. Don't press too hard though, the stuff in the phone feels pretty delicate.