There are currently no viruses for Mac OS X. I'm also not aware of any executable spyware (other than tracking cookies, which some might consider to be an invasion of privacy - although these are easy to purge by deleting your cookies).
The AV products currently being sold for the Mac scan mostly for Windows viruses. Your Mac can't be infected with them, but it's possible for you to transmit them to Windows users if you're sharing files. My personal opinion on this is that if you're a responsible Windows user, you should be running your own AV product.
The other half of the AV equation is that Mac AV products will monitor for virus-like behavior and attempt to thwart it. Those measures are typically ineffective and tend to be more hassle (and performance loss) than they're worth. And as I've said many times in the past, the first OS X virus will be HUGE news. Chances are you'll hear about it long before there is any immediate threat to your machine. When that happens (and I'm sure it's just a question of "when", since that will be a huge trophy for the scumbag who writes the first one), I'll consider a lightweight AV product - until then, I highly recommend just doing regular system backups with a product like SuperDuper. If and when the unthinkable happens, you can always just format your hard drive and restore from backup.
Having a good backup strategy is very important regardless of your reasons for doing so. Especially seeing as how modern hard drives seem to have higher failure rates than in the past.
Just my $0.02.