I like the value for money with SpamTitan. The VM appliance doesn't consume much resources. Spam tagging is good, managing the interface is straightforward and works. The appliance is very very stable. It never crashed in the last 8 years. Updating is straightforward, questions sent to support are answered fast.
I manage several different types op mailservers, postfix, hmailserver and MS Exchange. All e-mails in and out are sent through SpamTitan, so it works with all kinds of mailservers. It is really stable and it protects a lot of people.