Cyren was garbage when we used it. It would miss identify emails that my cat could have flagged as spam. Though, to be fair, MessageSniffer is not much better... Cyren had issues with their DC going down which would stall out mail, SM crashes etc. drove us away from Cyren to MessageSniffer.
We have a pop3 mailbox setup to send false negative spam emails to that Arm Research pulls down, I think it is just all for show, they pull down the emails, but nothing ever happens. I've been getting the same non-english spam mail for over a year now, reported every one of them, makes no difference.
We used to use an external SpamAssassin appliance that worked pretty well, but since SM took away the spam/ham folders, we cannot train SA anymore, which stinks :(