If it ain't broke, don't fix it. But if it is broke, then ifixit!
Dead home button and dying battery on an iphone 4 (and later a screen replacement). I got a new phone anyway, but wanted to keep the old device around for the kids - using it for games and as a pseudo-ipod Touch. Also used it as an opportunity to show the kids the innards of a highly engineered device, and how you don't need to instantly "throw something away and get a new one."
Toolkit, tray and repair guide from ifixit were great - changed out the home button and battery in almost no time, but damaged the LCD/digitizer connection cable in the process (the instructions said be careful you don't tear it -- they weren't lying!) Ordered a new screen from ifixit and got it all working good as new (for 2010) in no time.
There were just two points I struggled with
1) with the home button replacement, it took me a few minutes to realize I had to slide the end of the cable into the socket (going from step 33 to step 32 in the guide) A more close up photo or animation could've have been useful in the guide.
2) When reattaching the display to the frame, really take your time and make sure the digitizer/LCD cables are fed through the slot in the frame kink free. If they're not the same length don't move on to the next step (and hope the likely bend you introduced into the cable doesn't cause any problems.) Definitely don't pull the cable to make the length match -- that's how I tore mine.