I've used Linux on the PPC platform a lot of times. The reason I have used it is only because those Macs weren't supported anymore under OS X. Personally, if the Mac can run OS X without any problems or slowdowns, then I wouldn't bother putting Linux on it. However, if OS X is just to slow for normal use or just won't install at all, then Linux it is.
And now, with all the desktop friendly distros available (OpenSuSE 10.0, Mandriva 2006, Ubuntu/Kubuntu 5.10, Fedora Core 4, Yellow Dog Linux 4.x, Debian "sarge"), it's easier than ever to get Linux installed on Macs.
Of course, there are other considerations, such as whether you have a New World Mac or Old World Mac. An Old World Mac is a Power Mac that does not use Open Firmware but instead uses an actual hardware ROM chip. These require an installation of the Mac OS on a small HFS/HFS+ partition before you even install Linux. Because of this, you need to use a bootloader called BootX that runs on the Mac OS (9.x and below). To make it easier, BootX comes with an Extension that allows you to boot into Linux before loading all of the Mac OS Extensions and Control Panels (which can cause problems when booting into Linux). There is another bootloader called quik that doesn't require the Mac OS partition, but this can cause more problems than it's worth. Old World Macs include all PowerPC based Macs from the PPC 601 all the way up to the G3 in the beige Power Mac G3 and the WallStreet II PowerBooks.
New World Macs use Open Firmware, which means the ROM is loaded into RAM memory. Because of this, a Mac OS installation on a Mac OS partition isn't required, meaning that New World Power Macs can be used a Linux only machines. These Macs use Yaboot as their bootloader for Linux, which installs itself in Open Firmware. New World Power Macs include all PowerPC based Macintosh computers starting from the Lombard PowerBook, all iBooks, all iMacs, and the Blue and White Power Mac G3.
The other nice thing about Linux on the Mac is that it tends to support devices that wouldn't necessarily be detected or have driver support under OS 9 or OS X....at least that has been my experience with the Macs I have installed Linux on using PCI cards that were only meant for Windows.
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So while it's not as polished as Mac OS X, it does still have its perks.
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