The SuperDrive in my wife's G5 desktop machine has been failing to read some disks for a while now, and getting worse. Finally, it's become intollerable, so I had to replace it. I was worried that any replacement drive wouldn't have the "Apple supported" bit set on it, but as of Tiger and iLife '06 (at least), that concern is moot. That's not to say that the procedure is completely trouble-free. But with a couple of tricks, it can be done.
Most of what you need to know can be found in this helpful document at Apple's site. If you're replacing your optical drive with one you bought from Apple, then that's all there is to it.
In my case, however, I bought a Plextor 670A. When I got done installing it, the tray hit the inside of the G5 case why trying to eject. The problem is that the bezel on the front of the tray is too tall for the G5 case opening. This is likely the case with all 3rd party optical drives. The geniuses in industrial design made the case opening too small. Grumble.
Fortunately, the Plextor drive came with a beige tray and drive front bezel and instructions to swap it. The tray bezel that was too tall snapped right off, but left a "raw" looking edge. I could have just left it that way, but I took another look at the old Pioneer drive to see if there was a way to use that drive's old tray bezel instead.
It turns out that that tray bezel is just stuck on with adhesive! It was reasonably trivial to peel it off and stick it on the tray of the plextor. I crossed my fingers and reinstalled it all and sure enough - it's perfect. You'd never have any idea that it wasn't the factory drive installed in the machine.