There are plenty of viruses that can spread through a Mac, even if they don't affect them. Word viruses are a pain too.
I currently have Norton AV installed at a small business I do work for, it sucks, period. Especially on a network, NAV's auto-update sucks as it store it's update schedule on a per-user basis instead of being able to set it up with an admin account for everyone. And when the update runs, it's slow and takes over the screen.
I tried VirusBarrier before using NAV and it kept screwing up network shares, and now I find them questionable because they keep making up these new "viruses" that everyone should be worried about.
Virex sucked because of no auto-update, no on-access scanning, etc. It was very clumsy. I haven't tried the new version, but it's .Mac anyway and I've never like any MacAfee product for Windows either.
Sophos was pretty cool. I haven't look into pricing, but they cater to businesses only. Also, their defs are only updated every month or so, which seems odd, and I don't think their auto-update works unless it's running on a Windows server, someone still has to manually download the updates every month which is no good for people that can't even burn a CD or use email attachments and they have no in-office IT staff.
I need something for the company that requires no user interaction to work, background scanning, unobtrusive auto-updating, etc. I think I'm going to give ClamAV a try, nothing else really fits the bill.
It's sad they need any, they're an all-Mac business, but a few months ago they got swamped with email viruses from clients that use Windows and kept inadvertently sending them to other clients, it was a huge mess, important documents kept getting bounced back, etc. We ran a virus scan and found hundreds of infected files on every machine in the office.