The thing to bear in mind is that a lot of 'Mac Antivirus' software has overwhelmingly consisted of signatures to stop you passing on Windows malware, rather than necessarily much that will protect your system.

EDIT: Just to be clear, I'm not saying this was necessarily negligent. I'm saying that time will tell how good any of these packages really are - assuming of course that Mac starts to get real malware rather than what I'll call trickware. That prediction has come and gone with little to show for it in the past, but it may not last.