I'm only aware of one or two viruses for Mac, one which was only bundled with a pirated version of iWork... But it's easy enough to make a virus for a Mac... Infecting the system without a users interaction is tricky, however... Their implementation of Unix BSD and Mach makes a pretty damn solid secure system, though it pulls a couple of points in speed...
But, I couldn't care less about viruses... I never used antivirus on Windows, and I have no intentions of using it here... If something somehow managed to get in (Which so far haven't happened for me on any platform), I'm pretty sure I can remove it again.
The reason I use Mac OS X, and not some other *nix, is that OS X seems more complete in it's UI, while most graphical environment for Linux and BSD tend to be held together with band aid, if you know what I mean. :P Also, there's more applications available.
The reason I use a Mac, is because the quality seems to be way higher than your average laptop... And, it's a heck of a lot slimmer and prettier!
Cons: PRICE.
(Sorry, I wrote this on my G1, so it may have a lot of typos
)