Appeal of OS X for *nix developers?

For most purposes, OS X is Unix. Aside from Xcode (which I personally don't care for), there isn't really anything there to make it better or worse than any other Unix-like system for development.

Most of the typical tools, libraries, languages, and interfaces are there, you'll even be using GCC for C/C++ work. As long as you're not developing against Apple/OS X-specific interfaces like Cocoa, you are developing on Unix.

I use OS X because it just works, thus not interfering with my development, not because it has magical fairy dust that makes it better than any other Unix for development.

I love Apple as a dev platform because I get all the power of the *nix commandline as well as Apple's developers tools (XCode).

The additional software/hardware polish, and quality of third party software make it all that much more enjoyable.

Mac OS X is not better than a Unix environment, it is a Unix environment: http://www.opengroup.org/openbrand/certificates/1190p.pdf

I'd go as far as saying it's probably the most used Unix, considering it's also in all iPod Touch and iPhones.

(As far as I'm aware, Linux isn't a certified Unix, but I may be wrong, perhaps a distribution/vendor went through that process.)

Sometimes, for professional reasons, you just have to have tools that are compatible with what your boss or customers use. This often includes proprietary tools like MS Office, whether you want it or not (OpenOffice can't always deal perfectly with Word documents). OSX provides this intermediate ground, where the developers can also be users or closer to their user base.