I use Ubuntu "Hoary Hedgehog" Linux for my PowerPC box for the following reasons.
1) I got free, awesome-looking CD's shipped to me through their "Ship-it" program (almost typed shit twice).
2) It has apt-get by default and that made my life easy.
3) It works the hardware I have, and nothing had to be changed at all. My USB mouse and keyboard worked right out of the box, although iMac's can't use the old-world mac 4-pin keyboards even if we wanted, nor their mice.
I would, however, want to try out Slackintosh, the Macintosh Slackware, to see which I like better. Thankfully, I have a second Mac I could do this on so I wouldn't have to reformat! iago tells me Slackintosh is horribly out of date, but I think (and I believe he backed me up on this one, but don't shoot me if he didn't), that some software upgrades and a kernel upgrade and I'll be uptodate.