The machine itself is compatible, but good luck getting the display and sound to work. I whittled away hours trying to set it up, but never got anywhere with Ubuntu, MintPPC, or Debian. I got closest to success with Debian, where the only thing that didn't work right was the sound and fan control (after a couple minutes of uptime, the fans would ramp up to full blast for no reason and never settle down). That said, newer versions of MintPPC or Ubuntu may have better hardware support, so it's worth a try. There's also no flash player available at all for PPC Linux (last time I checked).
If your machine has less troublesome hardware, Linux works at least as well as, if not better than, OSX Leopard.
Gentoo works on PowerMac, no problem for fan control and sound. But the most important issue for me is of OpenGL things, when Nouveau drivers are used for Nvidia cards.
Ya, I had trouble getting linux on my MDD with 3 video card choices - either original nVidia GeForce4 MX or slight upgrade nVidia GeForceFX 5200 or ATI Radeon 9000.
What worked - Debian 6.0.9/10 with 2.6 kernel that supports the "nv" driver with nVidia cards. Newer distros (w/nouveau)=no go. (I don't think the nv driver does OpenGL, either)