Spamassassin is the last thing I use in my arsenal. It's too processor intensive. I use Mimedefang and sendmail checks as the first line of defense (spoofing, bad rcpt throttling, mail to system accounts, invalid helos, trustworthy RBL listings, etc.) On a typical day I *REJECT* about 5000 messages before going beyond 'HELO', 'MAIL FROM', and 'RCPT TO'. Of the rest that come though, I drop maybe 50 via spamassassin, and another 50 get flagged as spam. That's 100 things analyzed versus 5000.
If you are using nothing but SA and Bayes, you are doing it wrong.
I'd post this week's stats from my servers, but slashdot's junk character detector is a piece of shit.