My iPhone 3G got more and more erratic regarding battery life. It had to be charged very frequently, pretending to recharge from 0% to 100% in 7 minutes, it switched off when I got a call or when I was just about to kill that last green swine in Angry Birds and so on. Once it closed down, it wouldn't regain consciousness unless plugged to the charger. A friend urged me into replacing the battery - what did I have to lose?
I dutifully rebuilt MJ's working place on my desk, just to make sure I'd be prepared for anything. Wiping the sweat from my hand every 3 seconds I took the dive and started to turn the screws. It took roughly an hour - I was very very careful not to break anything. The hardest part was to pry out the old battery. I was so scrared to damage the phone, I almost cried whenever there was a cracking sound. But all went well in the end, and the battery came off with a last double-sided adhesive-sigh. The reassembly wasn't that exciting, but I began to feel very manly. And when I hit finally the power button and the silver apple greeted me, I started to think of setting up shop as an iPhone doctor. Three weeks in with the new battery, everything is still working well.
Go for it, it is definitely woth it. You will be rewarded with not exactly a new but at least your re-animated old iPhone. Plus you'll get that warm feeling of "I have made fire!"