I find myself wanting to use OS X more and more and windows less and less now. My main computer is a windows 7 desktop with 3 monitors but I feel like OS X is more polished and a lot more, eh, fun(?) to use.
I would love a desktop with OS X on it. Should get an iMac or a mac mini when i graduate.
It really depends on what you want to do with it. A Mac Mini is an absolutely solid My First Mac, especially for desktop PC owners who might already own a keyboard, mouse, and monitor. I wouldn't use it for any heavy gaming, though. If you're going to be doing that I'd jump right to the iMac.
Just keep in mind that not a lot of stuff inside these things is user replaceable (yes, even some of the iMacs), and more and more that includes RAM (Check their support site to see if the machine you're interested has official user-replaceable memory, then check ifixit.com to see what you can do with a screwdriver.)
On one hand, yes, this is annoying. On the other, Mac OS is really good at managing memory and Apple's memory prices aren't *blatantly* higher than what you'd get out of Newegg.
All that said, I still won't hesitate to recommend at least the Mini; if you can find last year's model refurbished on their site, that's even better as it should have user-accessible RAM slots.) It's the cheapest entry to point to owning a Mac. You can concentrate on acclimating yourself without feeling like you have to justify a nearly $2,000 purchase.