Using Clam AV on an AIX 5.1 Postfix mail server at the moment.
As for the effectiveness I cannot comment, we haven't had a virus problem, and I haven't looked at the AV logs yet to see if it stopped anything.
IBM Certified Confused - MQSeries
IBM Certified Flabbergasted - AIX 5 pSeries System Administration
MS Certified Windblows Rebooter