As MrZammler stated above, although the kernel bits are getting a little old it works fine. I ran Debian on my A4000T CSPPC for a while and found the speed to be acceptable. The only somewhat tricky part is getting the damn thing to boot in the first place. Once you're past that hurdle it's pretty much gravy (if you've used Linux, particularly Debian, for a while). It's been a while but I don't think Mediator is supported, it can use native Amiga chipset though.
You should find
this page on the Debian project website to be helpful. It also contains a link to the Linux-APUS website. You might also find
packages.debian.org useful if you want to see if a particular software package is available precompiled for PPC.
*edit Above I stated the kernel bits were old. It looks like there was work being done on 2.6 somewhat recently judging from CVS at the Linux-APUS project.
