So, I had this old iPhone 3G back from 2007 that I gave to my mother, but a couple of years ago I happened to fall while skiing on the Alps and the display got damaged (it had an expanding stripe of broken pixels) by the shock and/or snow getting inside. Apart from that, the battery didn't last as long as new, so I finally decided to take it apart and give it a good fix!
Removing the display assembly was really easy, apart from the last cable (I missed the bit of the guide about having to open the plastic clip before removing the cable!). I also found that the right screwdriver to remove the screws was the Philips #000, at least for me (whereas in all guides they recommended a Philips #00). Replacing the battery was much more fiddly, but in the end I made it. If I should do that again, it would be much easier than the first time.
Be careful not to break the golden ground pin on the side when removing the motherboard.