Macintosh is a solid OS, I've got a machine with OS 10.3.9 on it. I haven't used it in a while though. My biggest complaint about Mac OS (correct me if this has been changed in newer versions) is that it hides so much from the user. For example, you cannot log in as root. That kind of thing bothers me. Not that Windows doesn't hide things, but it's easier to figure out issues with Windows. Then again, you could also argue that Mac OS has less issues to begin with.
EDIT: Just realized, this is a pretty old thread. I didn't notice until it was too late. Oh well.