Mac/Linux for system administration tasks
Like many other linux users, I switched to a Mac for my daily use a few years back. The main reason I did this is I wanted to use a laptop, and I got so sick fo dealing with things like suspend and resume on linux laptops. I'm sure things have improved, but the bottom line is that my macbook generally just works without me having to screw with it.
All the standard unix tools are available on the mac, and the virtual desktop support is decent. No focus-follow-mouse, which took me a while to get used to.
My company has an exchange server and the exchange integration in Snow Leopard works just fine. Finally I can schedule meetings and see everyone's availability. I do still run mutt to read my mail.
If there's any unix utility you don't have installed, a quick run of macports will install it. I use the Aquamacs editor heavily, and it's a nice blend of emacs with mac functionality.
I think the key thing is that most system administrators spend their time logged in to other machines anyway. Give me terminal.app and firefox and I have pretty much everything I need for that. I will probably try running linux on a laptop again in a few years (especially if I have to buy my own laptop) but for now I'm very content with my mac.
The relative suitability of any workstation OS depends on what your admin responsibilities demand of you, but I find that the Mac platform provides the best flexibility: you get what in my opinion is a much more polished user experience from OS X than Windows, but the many virtualization options available, particularly those like VMWare Fusion which integrate with the window manager, mean you won't be stuck if you do end up needing to run a Windows- or Linux-specific tool.
Well I use a Mac as well as a Windows machine at work to do my sysadmin tasks. What sort of pros and cons are you looking for?
To be honest, I'd question the value of mac as a sysadmin platform for a business that wasn't using other macs and hence needed the Mac OSX specific admin tools. If you plan to use it for your general work environment, e.g. office tools, email, browing, etc. then I find it a very pleasant environment to work in.
That aside it will operate as a reasonably standard Unix environment with the tools you'd expect, you can add things like VNC and RDP viewers to it, etc.