Mail Scanning and Anti Virus

I currently evaluating a system to scan our email for Viruses and Spam.
I'm using Postfix and plan on using Amavis. I would like to hear about
your experiences with Virus/Spam scanners. Any suggestions would be
greatly appreciated.

I would prefer to us an open source solution like Clam Antivirus or
Open Source Antivirus. Has anyone used these scanners and what do you
think?

Chris Cowan