It's advantage is that... well it is Apple. Which means easy to use, and compatable with desktop macs if you have a lot of those. Most of the advantages of Unix and most of the advantages of Apple. It's a win win situation, ecxept of course for your pocketbook. But usually it's worth it.
Performance-wise OS X isn't as fast or powerful as a linux or bsd server, but it makes it up for a lot of people by having corporate support. When we were having troubles with a aspect of networking OS X we tried calling apple's tech support, and they couldn't help us, eventually they got thru to the actual guy that was head developer of the specific part that we were having trouble with. I like that kinda service.
Of course if you use linux mailing lists you are usually communicating at least inderectly with the actual developers, and they may read your e-mail if it is good enough question (not answered a dozen times for other people, or by other people, and not in the FAQ or the documentation). But then again you can't get help-on-demand like you can by paying for it on a corporation's tech support.