Why is sendmail bad?

[Kenneth Porter]

There seems to be a lot of dissatisfaction with sendmail in the call to 
replace it with Exim or Postfix, but I didn't see any specifics about why 
people object to it. Anyone care to give details?

I'm using sendmail together with MIMEDefang to run SpamAssassin and ClamAV 
against messages during the SMTP transaction so that I can reject obvious 
spam and viruses before they're committed to a queue. MIMEDefang uses a 
user-edited Perl script to establish the exact rules for acceptance, making 
the system very flexible. How hard is that to set up in Postfix or Exim?