OS X is the main reason for me. Overall, I find it the most well-rounded, feature-packed and pleasant of the OS's I've used regularly (the others being Windows 98, 2000 and XP, and Solaris). Some reasons:
* A user interface that's consistent, "ergonomic" (if such a word can apply to GUIs), and beautiful. And yet, the 'eye candy' somehow manages to look respectable and mature, rather than garish and unnecessary.
* UNIX under the hood! Seriously, without this, in my line of work, a Mac would be a pretty but ultimately useless toy. With it, on the other hand, any complaint I might have had about the Mac being a proprietary 'niche' OS with too little software available for it suddenly becomes moot. Nowadays, I tend to look at Windows as the disadvantaged 'niche' system.
* The fact that the last two bullet points are
combined in one operating system. That deserves its own point because in the past it's always been a choice between having a decent, user-centric GUI and shabby-to-nonexistent tools for programmers (as in Windows) or having UNIX with some lame, knockoff GUI on top of it (Linux, Solaris, etc). That OS X manages to not only have the best of both worlds without going to the extremes of either, but that on top of that the GUI is
far and away the best one out there, puts it wayyy ahead of the pack.
* Built in programs I use daily: Expose, Safari, XCode, Stickies
Built in equivalents on the other OSs I use: uhhhh.......
* A minor thing, but I still find it neat that I can just drop an app into /Applications and it's "installed" and ready to go
* Lots of popular open source software and libraries built in. Heck I hear Tiger even comes with wxperl preinstalled! Unlike Linux, of course, where you're expected to compile everything yourself (ridiculous. This is 2005.), or Windows, where half of it isn't even available for the OS
* The fact that you get all of the above without losing compatibility with most Windows file formats and PC-compatible devices.
As for Macs themselves (regardless of the OS), I can't really think of a reason other than that they're so PURDY!
On the other hand there are the more immediate, personal reasons:
* Since I work with Macs, and I find myself doing more and more work at home, it would really help to have one here. I want to try Remote Desktop, too. Can't really do that on my PC. Also need to occasionally play 'tech support' to my Mac-addicted sister, over the phone. 'Be nice if I had an actual machine to refer to.
* I have access to quite a bit of pro Mac software. I'd.... hate to see it go to waste...

also in that vein, I'm interested in trying out Apple's DVD authoring software (though I guess I'd have to actually
buy that)
* I have absolutely no reason whatsoever to get another PC, nor can I see any real value in replacing any of my current ones with a newer one. What am I gaining? The possibility that I might have good enough hardware to run Longhorn, which may or may not suck and may or may not ever be released? Please. With a Mac I'm gaining OS X, and that's worth the price of admission right there.
* I'm running a lot of unix software at home that has been 'ported' to Windows, but the deficiencies of some of those ports are starting to convince me that I really need to be running a real unix system. Or Cygwin, but I'm not a big fan of cygwin. Again, for the above reasons, os x is the obvious solution to this problem.
* The introduction of the Mac Mini has finally made getting a Mac a realistic possibility for me.