We've been running Antigen for Exchange 5.5 and it adds the ability for subject line filtering as well as pretty good anti-virus functionality.
Unfortunately, we got hit so bad from all of the SoBig traffic that our server just couldnt keep up with incoming flow. We have added a eSafe e-mail gateway that scans everything before it comes into our Exchange server and that seems to be handling everything pretty well, except for all the bandwidth that gets eaten up.