My 3 year old AT&T iPhone 4 had a temperamental Home Button and a battery that had lost a lot of its capacity. I decided to replace the two components to extend the life of the iphone until the next generation is released (iPhone 5s? iPhone 6?)
The repair was how-to story of what NOT to do. I took great care during assembly, but I wasn't careful enough. I didn't realize that while trying to separate the screen from the frame, I was pulling against the two ribbon cables. I ended up damaging the longer of the two cables which is responsible for the digitizer function. The damage either occurred during the separation or the re-assembly. Be absolutely sure that the longer cable is fed through all the way and not folded upon itself. Upon re-assembly I had a perfectly functioning Home Button and a new battery but a phone that had no digitizing function...essentially useless. I didn't have time to order a new screen so I picked up up on CraigsList that same day for $35...just to see if that was indeed the problem. It was. I reassembled, only to find that I had abused the cellular antenna connector. It wouldn't snap down on the logic board. Now I had a phone with a perfectly working Home Button, a strong battery, a restored digitizer screen...but no cell service. Argh! I ordered a replacement cellular antenna from ifixit.org which includes a new connector. It popped right in. Now all systems are go. However, during the dozen or so disassembles, I lost interest in being careful with maintaining which of the infinitesimally small screws went where. I have a handful left over. ;-) I concluded that I would never buy a used phone unless I was 100% sure that it had not been opened.
When reinstalling the screen, triple check to make sure that the longer, thinner screen ribbon cable is pulled through fully and not folded onto itself.