Knock on wood, but I have never heard of an OS X virus, and viruses in the Classic Mac OS have always been very rare birds. I've been using Macs since 1984, and I've never seen a real, live Mac virus. I think Mac anti-virus software survives on the FUD that spills over from the Windows side.
I once had a virus that affected MS Word. It blocked me from saving changing in a Word document. Didn't do anything else to my Mac, though I was told it was a bit nastier to Windows machines. That was about four years ago.
I've been using Macs now for about 15 years and that's the only time I've encountered one.
However, about 18 months ago I moved to a new employer who uses a Windows network. My computer at work has been infected twice, and it does have a regularly updated antivirus program.
That's been my experience -- except the part about getting a Mac virus
My work pc has been crippled more times than I can count on 2 hands because of viruses. And this is a company that not only has daily virus updates, there are about 10 people working around-the-clock maintaining the systems. Every time they have to service my computer I say "Why not switch us all to Macs?" I always get a with that remark. After awhile I stopped saying that because it dawned on me: If we switched to Macs, they'd be out of jobs. No wonder they're pissed with that suggestion!
Still trying to track down, from that Sophos virus report from a week or so ago, which virus (by name) it was that came in at #78 - the highest incidence virus that infected the Mac. No indication in the article whether it was under X or pre-X...