I am sure there will be others that chime on this topic too! 99% of mac owners do not run a separate antivirus program. The vast majority of "malware" - 99% - is written for Windows. Safe computing goes a long way to avoid problems. Even with the iWorm it seems you had to be asking for it by downloading from questionable sites.
Apple has an in-house antivirus called xProtect built in that is very quiet, updates in the background and has already updated to confront the new iWorm threat.
That said, I have just earned my first year pin for owning Macs. I started this venture installing a wide variety of antivirus products. I just had to try them all! I do have strong opinions on clamXav, avast, kaspersky, and comodo some work better than others. As to do they stay updated on Mac viruses, rumor is they don't.
The bottom line is this - of all that I tried the only thing they ever caught were infected emails carrying virus that were written for Windows systems. It is very hard for a recovering Windows-aholic to let go of antivirus but in all honesty it isn't needed.
I suggest you do what I do - read, read, read. This forum is awesome for keeping up with what is going on. You will quickly figure out who really knows the inside info that will be relevant to what you need to keep yourself informed.
Good luck with your conversion! If you are just experiencing OS X for the first time there will be a learning curve so give it time, ask questions here, and you will soon be an expert.
Lisa