The home button and rear facing camera were not working. The rear facing camera would take one picture and everything would freeze up, requiring the phone to be rebooted. The home button required frequent presses before being detected. I want to use the camera in some "little robot" projects I'm working on, so the camera was essential.
The rear-facing camera was a cinch. The home button was a bit more involved. I didn't have to peel the metal frame off the case, but pretty much everything else comes apart. It did take nearly an hour to get it all apart, replaced and reassembled. It took a long time to boot, I assume because the battery had been disconnected, and I was sure I had bricked it. Surprise - it's back in service and ready to be attached to the next project!
I used a magnetic parts mat and indelible marker to label each screw as it came out. Rule one - you must do this or you will never get the right screws back in the right places! The mat saved me a couple times when I dropped parts. It catches them, rather than letting them bounce away. Rule two - don't do this over carpet - if you drop one of these screws, it's gone. :)
iFixit Opening Tool
iPhone 4 Rear Camera
iPhone 4 (GSM/AT&T) Front Camera
iPhone 4 Home Button Assembly