My 3GS battery life was slowly getting worse and worse to the point where it would only last me half the day before I had to recharge it. This was only using the phone for texting and general internet browsing...if I actually had to phone someone then the battery would likely die between 4-5 hours of use. I was going to send it into the Apple store, but decided I was more tech savvy than that and could replace the battery myself (with the help of iFixit of course).
Repair went very smooth. The picture tutorials offered by iFixit are fantastic! I've had to make tutorials for other things so I understand the difficulty and time that goes into making good, coherent tutorials. The process took a little over about 2 hours, which is longer than it should have taken, but this was due to me studying the insides of my iphone because I've never taken one apart before.
The spudger is ridiculously useful. If you never use it again to disassemble your phone you can absolutely use it for other electronic endeavors where tiny baby hands are needed but unavailable due to restrictions on child labor. I was hesitant on ordering one since it looked like a basic piece of plastic, but it is most definitely worth it!