Mac OS X. The operating system is, after all, the foundation, not the entire structure. Mac OS X will not boot from a secondary drive, it has to boot from the primary hard drive. It is easy however to boot from Mac OS 9 after installing Mac OS X, but the computer is much more temperamental. Slowly, I am learning which applications work with the new operating system and which are problematic. Mac OS X really is a beta, in the truest sense of the word.
Parts are unfinished.
Airport does not work, for example, nor does Mac OS X support many printers. It does not take long after switching to find something that cannot be done, so do not depend on Mac OS X beyond learning and testing. Unless of course you would prefer to strap on the afterburners and explore the wild blue yonder.