The oldest iBook actually has the best build quality, I think. The keyboard feels good, and none of the keys have rubbed off. It is fast and solid. However, since it is so old, I decided to use OS 9 as the primary OS. We inherited a bunch of old educational CD's from the 90's and the iBook plays almost all of them better than the PPC-based Mac mini in the den. Perhaps the biggest drawback? The smell of burning plastic that is emitted from the possibly-not-covered-by-recall motherboard. Oops.
The newest iBook is zippy enough to handle video from apps like VLC (for the playlists) without a hitch, so we're using it as a video jukebox of sorts. Connected to the network I can either stream or move videos to it for the kids, plus it runs all the OS X educational software we've got. Oh, and TuxPaint, which also runs on my personal machine in case they both want to draw at the same time. The working optical drive on the 1.2GHz came in handy at the beach recently, when the TV in the kids room didn't work. So we used the iBook as a DVD player.
Of course, if you work in primary education this is probably a snoozefest to you. Apple sold quite a few (though not nearly enough) iBooks to the educational markets back in the day. So it should come as no surprise that my wife and I recycle our old work machines as kid machines. What makes the iBook so special is the ability to run OS 9 natively and the form factor. Sure, iMacs are great, but you can't take them on vacation. The old clamshells are OK as well, but a little bulky. Then again, maybe I just can't bear to throw anything away.
UPDATE: Added a gallery.