I've been using Mozilla 1.3's spam filtering facility for a month or two now, and after the first week of somewhat iffy training elapsed, I've been making grandiose claims to anyone who'll listen concerning its effectiveness at correctly identifying and filtering my junk mail. I get a lot of spam -- I would guess it constitutes over 50% of my email -- and I really have been absolutely stupefied at how well Mozilla's solved this problem for me.
But I've only been able to guess at its statistical effectiveness, because it doesn't keep stats. I have no real idea what percentage of my email is spam, or how much of that spam Mozilla intercepts... I can only guess wildly. So I'm going to see how well it does, statistically, over the course of a typical day (6pm–6pm), and tomorrow I'll post the results.