SpamTitan is easy to setup and maintain. The detection rate is fabulous. When we initially setup SpamTitan as we were getting tons of false-positives but this subsided after several weeks of use after which time I assume the system started to pickup on the types of emails that we consider legit.
Once we got through the growing pains we didn't have to field many helpdesk tickets from users getting use to this software coming from our previous spam filter. The other big benefit is that our previous spam filter service provider was about to double our yearly rate so SpamTitan software ended up having a better spam detection rate and was also less expensive.