Here's a
PC World article on the subject.
Personally I don't use one on my iMac - just keep my eyes peeled in the media for reports. The media just goes into a feeding frenzy whenever a new 'virus' or 'trojan' is detected. And Apple's built in protection has worked well for me.
Beware - if you are running Windows, or using MS Office products you are not protected from malware affecting them. For my needs the built in Apple products work just fine.