Spammers were flooding my users and traditional filters such as SpamAsssassin were ineffective. I tried traditional DNS-baed blocklists, but since we go through our hosting service's DNS servers, we were blocked by the best services due to volume.
I was skeptical about MagicSpam but, once I tweaked the recommended blocklists, it has been very effective. Not 100%, but it brought the spam down to a level where my users stopped complaining. On a typical day, 75-80% of incoming email is caught as spam by MagicSpam!