If you're like me and can keep a clean, lean, XP machine, I find myself missing the start menu, the start bar, iTunes and WMP controls in the start bar, and that dock drives me batty at times. Not that it's a bad idea, but when I have 10 instances of notepad open, I want to be able to select the one I want without using expose or anything.
But, on the other hand, if your computer is always getting mucked up with spyware and such, you'll probably become a fanatic about OS X, because it is almost impossible to break... and if you do manage to break it, 50% of the time it can be fixed without a reinstall, and the other 50% of the time, it's one of the most painless reinstallations you'll ever meet. Easier than installing a program on XP at times..