Multiple Spam Filters
I have a nice system, where:
1) Spamassassin tags all mail (without bayesian filtering) but does not reject anything.
2) Qmail hands off mail to procmail for delivery after running spamassassin.
3) SPASTIC runs through procmail and (after processing whitelists) rejects spamassassin tagged email (over a certain level), plus running most of the other SPASTIC tests. all the spam goes in one box, the rest goes in my Inbox (or a subfolder in the case of some lists) I maintain an HTML archive of spam, and browse it weekly, just to check nothing important has been missed.
I find this works well for a user who does not have access to the spamassassin setups (or userprefs enabled for spamassassin on my shellhost). I can have whitelists so spam sent from my friends doesn't get rejected, plus i can blacklist addresses.
This method cuts my spam from 200ish a week down to 1 or 2, and the false positives in the 6 months or so i have been running the combination amounts to single digits.
I concluded that combining spam filters is a good idea - multiple levels rocks!