Despite the large number of people who say that (1) there are no viruses targeting the Mac and (2) even if there were, the Mac's near perfection would prevent them from infecting any machines, I believe that there is stuff out there and it's just a matter of time before someone develops a really effective bit of malware targeting Macs.* It's debatable whether one needs an antivirus program on a Mac, but on a cost/benefit basis, it seems to me that one ought to be protected.
I'm currently testing the free Home Edition for the Mac from Sophos. It seems to have a fairly light footprint and doesn't seem to interfere with normal processing. I did tell the auto-protect subsystem to ignore the VMWare files. There's no point in scanning those, and I'm using a Windows AV program inside the Windows XP virtual machine.
I had previously tried the Norton Internet Security suite. That stayed on my computer for about 12 hours. It was intrusive and it noticeably slowed the system. It was difficult to turn off the parts I didn't want (e.g., firewall) and conflicted with TimeMachine and mail. These problems are not present with Sophos.
The biggest plus I see is that Sophos is very up front about supporting an open user forum. The first step in the installation takes you to the user forums so you can see if others are having problems before installing. I learned that there was a problem with TimeMachine, but it was fixed in a recent update. That's good stuff to know. It also demonstrates that Sophos is staying on top of the user comments and complaints.
* The folks at Sophos, who clearly have something to gain, have several videos detailing existing threats to the Mac environment. Other warnings are and here. According to , "The reasons why Macs don’t get many viruses are as much based on luck and market conditions, as they are on inherent security." at GigaOm