I've been watching this one. I don't think it's ready for prime time quite yet, but it's getting close. I hear that the next version will have some interesting new features.
In any case, anti-virus software of some type is definitely worth running. Macs are better than Windows as far as viruses go, but they aren't invincible. It's only a matter of time before a virus which can infect Macs is written, and even those viruses which can't infect a Mac can still exist on a Mac, effectively making it a carrier.