I have a pair of internet relays in a DMZ that forward for firewalled local mail servers. MimeDefang runs virus scans on all mail and spamassassin on inbound mail, first checking that the user(s) actually exist on the destination host with the included md_check_against_smtp_server function. I don't think postfix offers that option, but I'd be pleasantly surprised if it can.
not the RHEL4 (therefore Centos 4) one. postfix does offer this option now.
It's real unique advantage is decades-long consistency and improvement. With the milter interface you get the ability to add any additional processing you want in the language of your choice - and with MimeDefang pretty much all of that is already done for you. If you are interested in why it is a good approach, there is a very detailed description if you follow the 'slides' link at the top of http://www.mimedefang.com.
I'll probably wait for postfix + milter support to become stable and take the advantages of postfix over sendmail if I need the features provided by mimedefang.