For a Unix sysadmin, it's wonderful. It gives you an easily usable desktop OS where you can run MS Office if need be, but also gives you tools to make Unix admin easy. Try scripting something on a Windows machine. Of course, you can always log into some other machine to do what you need, but that's always a bigger pain than just using your local machine.

Virtually everything that runs on a Unix machine will run on MacOSX, plus, most everything that you need on those occasions where you "need" a Windows machine will run under MacOS, and those few times where even that won't work (Outlook, I'm looking in your direction), you can run it under Parallels. (I don't use Parallels ATM, as I'm still stuck on PPC Macs, but I have yet to hear anyone say anything bad about it.)