Ubuntu is very good for x86 and simmilar systems but has been seriously having some issues on newer PPC Apples.
For PPC systems, you should probably look at SuSE (casual) or Gentoo (some effort required, but works very well once set up)
I have a Powerbook G7 17" (HR) 5,9 - 1.67GHz w/ 2GB DDR2-533 and have been through the ringer with Ubuntu. Right off the bat, I was using BCM43XX (the open source airport driver) at which point it worked flawlessly in Ubuntu, however when I rebooted into OS X, I could no longer see that an airport card existed. To resolve that, I needed to CMD-OPT-P-R and then boot into OS X. Then, it worked just fine, but when I rebooted into Ubuntu, my Airport disapeared from OSX on me again.
Then, there was an issue with pbbuttonsd, which prevented me from being able to eject a disk from my superdrive, or change my volume at all. For volume, if I tried to change through the menubar in Gnome, it would change for a fraction of a second and then go back because PBBUTTONSD would keep resetting it to what it wanted.
With Gentoo (it took quite a bit of research though), I got everything working flawlessly. The initial problem is that Gentoo is a build-it-yourself linux. That being the case, I needed to track down that snd-aoa was the kernel module needed to work with the "Toonie" soundcard used in my Powerbook, that bcm43xx only works reliably if you have a special kernel option chosen in the Drivers section (one that Ubuntu 6.06 PPC had opposite at the initial release).
Hopefully Ubuntu has fixed those issues by now, but atleast on PPC, it has not faired well for me and I am not that interested in going out of my way to give it another chance any time soon.
BTW: 24 Hours until WWDC!