After some painstaking efforts, I got Kubuntu running on my PPC Powerbook G4 a few months ago, and recently upgraded to Hardy. After 7.04 or so, Ubuntu stopped officially supporting the PPC platform and it really is starting to show. Configuration and installation issues aside, the computer runs hot, the fans are always blaring, and the speed is remarkably slow. These are probably the result of the kernel using non-specific drivers, the lack of a PPC nVidia driver that isn't terrible, and generic configuration without any real testing.
I don't really like OS X, but given that I can't get an x86-based laptop any time soon, I was hoping to run Linux on PPC but it seems I'm barking up the wrong tree. Gentoo is out of the question given the struggle this poor old processor goes through compiling the smallest of things. Linux is a good alternative, but perhaps the PPC world isn't a good place to be.