My boss has an iPhone 3G. Battery was wasted. He asked for a new phone, but his upgrade was used for someone else. Since he only uses it for phone and email, no need to upgrade anyway. So, we decided to attempt to replace the battery so he can keep using the 3G which is more than capable for his use.
The repair pretty much went as described in the guide. First time inside an iPhone but not unfamiliar with the innards of electronics. I took it slow and stuck to the guide. Took about an hour with interruptions. Only issue I had was that I didn't seat one of the cables fully when reassembling. The earpiece did not work when testing. Opened it back up and reseated the cables and it was fine.
Don't be afraid to cautiously use a little force to pry the battery from the adhesive holding it to the back of the phone.