After Virex did more damage to me by deleting multiple unread emails, and then got bought out by McAfee, I simply don't trust anything McAfee says.
The version of Virex I was using detected malware in incoming email and was supposed to kill the malware. What it did was kill the entire post, which meant that I never saw it and therefore could not update my filters to get it. Worse, if I checked mail and there were 10 posts in the queue and post #3 had malware, then Virex would delete post #3... and 4, and 5, and 6, and 7, and 8, and 9, and 10, even though the other posts did NOT have malware. Worse, Virex would not say that it was deleting all the posts. Worse, at the time I was mostly using POP, so all those posts were gone for good. Worse, Virex would flag the malware that caused it to delete post #3, and 99.9% of the time it was for Windows malware, and where it wasn't for Windows malware it was for something I had protection against. Shortly after I found out what Virex was doing, it was gone for good. Virex was comparatively benign compared to McAfee and Norton on Windows, both of which deleted system files causing the machines in question to not boot. On several occasions the easiest solution to the problem was to reformat and reinstall. It was a good thing that I had good backups or I would have been quite annoyed.
I have been using Macs since 1984. I have been using DOS since 1981. I have been using Windows since 1985. Virex has done more damage than all Mac malware I have ever encountered. McAfee and Norton on Windows have done more damage than all DOS and Windows malware I have ever encountered.
Sophos and ClamXAV haven't caused the problems that Virex and others have, but they slow the systems they're on down and the only benefit is that they catch swarms of Windows malware which can't affect the Macs anyway.
There is currently no major malware threat for Macs. There used to be lots of major Mac malware, but that time passed more than 20 years ago.
All that vendors like Intego, McAfee, and Norton are doing is trying to drum up business using scare tactics to make people afraid of a non-existent threat.