I installed Yellow Dog Linux with no problems whatsoever on my Mac. But it dosn't support my videocard (GeForce 3) so I run it with my secondary ATI 7000 card whenever I get the inclination.
It works with the GeForce 3, but the colors are all off. Other than that I find YDL to be very stable and easy to work with. Not easyer than OSX, but pretty simple none the less.
The install is quick and painless and has good documentation.
Another distro I tried was Mandrake which also has a very easy installer, tho I could never get it to run.
The only major problem I encounterd was using Yaboot, the standard bootloader. It was a pain in the ass to get of the Harddrive. After fiddlin for what felt like forever I could not wipe it.
Eventually this conflicted with OS9's bootloader which prevented me from logging back in to classic. This caused me so much trouble that I just called in my warantie and got a new hardrive
In any case, YDL is the way to go.