NEMX Power Tools for Exchange works really well for us. $495 one time fee, good tech support, installs on 5.5 AND DOESN'T REQUIRE A REBOOT even on an NT4 server.
Sits on the box and can be configured by you for filtering by reverse blacklists, subject words, addresses, and other items. I have been tweaking the rules for the last two weeks as we have decided not to yet purchase the content filter (searches content) for additional 295.
Best of all, trial version is fully functional for 30 days with free support always.
I have been using for last two weeks and been getting lots of praise from the users.
I have been reviewing all yanked spam for false positives and would recommend you do this for a while with whatever product you use. Luckily we have only 100 mailboxes so I only have to review about 1000-2000 spams per day.