Heh... I'm gonna have to agree with Lycander here. I use OS X more than I use Linux, but using Linux on an old 8500 with a 400MHz G3 is MUCH more responsive than using OS X on a 500MHz G4.
The only complaint that I have is that Linux is put together by thousands of people and the UI is not anywhere NEAR consistent. It's maturing into one hell of a great desktop OS, but still is not ready for casual users who are used to booting from a CD, initiating an install, then getting down to business and just using the computer. Hell, making an alias on your Desktop to launch Mozilla is a multi-step process and would be confusing to the casual user.
In spite of that, I would still recommend it highly. If you can configure a Linux desktop machine and install updated software (without the use of yum or apt-get), you've got a good understanding of UNIX and how it behaves. Once properly configured, my Linux machine seems to run faster and perform better than my OS X machine -- it's not bogged down by the UI and you've got TOTAL control over how EVERYTHING works.