Because I'm bored, I just ran some numbers on my email account, comparing the amount of mail SpamAssassin correctly identified as spam to what it incorrectly identified as legit (and I had to flag as spam) and what it incorrectly identified as spam (and I had to go looking for in the spam folder).
The numbers are not terribly surprising. (Excuse the screenshot, but I'm not quite bored enough to format a table for an MT entry. Also, my foot hurts.)
So, SpamAssassin correctly identifies 87.5% of my mail as spam, goofs (in some way or another) on another 1.4%, leaving me with 11.2% of my incoming email as legit.
A couple more notes:
I have a procmail script that backs everything up to a folder before handing the message off to SpamAssassin (that's where I get the total message count).
SpamAssassin 2.63 is installed on Neophi.
I have adjusted SpamAssasin's sensitivity up a tiny bit, but not much at all.
The 3 false-positives are the ones I am aware (when I was expecting something that didn't show up in the inbox). I basically never comb the Backup folder or any of the spam folders looking for something unless I get to wonder "WTF?" about a missing message. That means there's a potential for lost mail from unknown correspondents.