1. They're awfully purty
2. The picture quality/monitors look better than any I've ever seen
3. The OS looks awfully purty, clean, and orderly
4. The Apple reputation for stability, reliability, clean design, programs that work seamlessly with each other, and security
5. They are plenty of computer for me and have top notch components
6. They come with software that's actually very very useful, fun, and easy to use
7. Apple seems to put a significant amount more effort into designing a well thought out computer than other manufacturers
8. I've never had a Mac before and I just wanted to try one
This about sums it up for me as well. My XP was in need of a complete reload having been run for a few years without one and there's just no other monitor out there like the 27" iMac, that I have seen.
Viruses, stability, looks, being different, and really just learning something new. Sometimes I sit there a bit lost, but it's all fun to discover a new method or a new program. Tweakability has been a nice bonus. It just works, and it's just fun.