While it is true that OS X is virtually virus free, and likely to be so, I would consider it unwise not to have virus scanning software. Unless you never have to interact with Windows users, there is always a possibility that one may unintentionally infect Windows users.
I used both Symantec and McAfee (.Mac), but based on Macworld's recent review of Intego's VirusBarrier, I would have to say that it is the best Mac virus scanning software out there. Symantec is in need of an update, and Virex (I thought) was a bit slow.