kSpam works well, but needs TLC from an admin POV. The UI should also be enhanced. it's open source though so that may be coming soon.

SpamSentinel has a nice UI, works well, but crashes our server after a short time, and I couldn't get the reporting options installed. It does allow users to monitor their incoming spam.

MIMEShield has a slightly less pretty UI than SpamSentinel, but has excellent reporting facilities, and all allows users to control spam.

MailEssentials needed the MS SMTP server to run, which was no great drama, but the Bayesian analysis wouldn't import HAM.

So we're still running kSpam.....