My problem with Mac anti-virus solutions run specifically to protect the Mac is that I don't understand what they're scanning for.
Traditionally, anti-virus programs work by one of two different methods. The first method is by downloading a frequently updated database of known viruses. Then, the scan engine will constantly scan for the signatures of known viruses.
Since there are no known viruses for OS X, there are no signatures to scan for.
The other method is known as heuristics. Heuristics scanning is a technology to scan for virus-like behavior and attempt to block it. This leads to major overhead as the AV program constantly monitors everything every program on your system does. It can also interfere with normal operation of your system, creating problems where they wouldn't otherwise exist.
Brown Study makes an excellent point - let the Windows users worry about scanning for their own viruses. That is part of the liability of running Windows. When the first Mac virus finally does arrive, it will be extensively covered by the media. You'll likely see the news before you see the virus. I'll worry about AV at that time, but there's no need to suffer the performance degradation associated with AV products until that time.