How do I block viruses using qmail?

This is one of the most asked questions on the qmail mailing list...and every list commentator has a different answer. Please allow me to list a few of the most suggested options:

Source scanners:

1. Jason Harr's qmail-scanner package

2. The AMaViS scanner

3. Russell Nelson's patch for qmail-smtpd,

Commercial Scanners:

1. The NetCleanse antivirus scanner

2. The Gwavix antivirus scanner

3. RAV Antivirus

4. Central Command's Vexira MailArmor

I would tend to recommend NetCleanse and Gwaxix.