This may be too drastic a conclusion -- but my first and only experience with a SPAM filter was a disaster. I installed an eval. copy of the IHateSpam and was quite pleased with the way it integrated with my Outlook client.
However, after a week of using it, I found myself spending more time with junk mail. To its credit, IHateSpam identified around 80% of the mail correctly but the remaining 20% gave me more headache. I found myself spending way too much time rummaging the Quarantine folder for legitimate mail -- more than it took me to delete SPAM manually.
Luckily, I was able to cleanly uninstall IHateSpam!