It's pretty good, especially after I've trained it with enough spam & ham emails to turn on its personal ruleset - probably not as good as Google spam filter, but at least I have some visibility into why given message was misclassified.
Additionally, you can read the email's source transmission. That way you can read the SpamAssassin headers and see exactly why it was flagged ham or spam. It's been a pretty indispensable tool for email testing for my personal projects for how nifty it is.
