My first Mac OS X experience was with the public beta version. We chose to purchase a second hard disk, and install OS X there, leaving our original disk untouched. After rebuilding that machine I vowed never to do this again. Well, emotions settle down, and a week later I tried the installation on my personal iMac, this time sharing the same drive. It worked fairly well, and I could boot from one or the other operating system freely.
OS X installs very cleanly.
In the meantime you get to experience the benefits of OS X in everything else you do. For OS 8 users, the deal is even sweeter. The Mac OS X kit includes three CDs, one of which is a native OS 9.1 disk. I think that's a shame, because the default for most applications is to install into the root folder. Otherwise you'll find that, under OS X, your hard drive will bear an uncanny resemblance to your dormitory locker.