Just set this up yesterday, rather than my previous configuration of Exim with a SpamAssassin router. It means I can now use a simple grep on the exim log file to see the ratio of spam (and other unwanted stuff like virii) to genuine mail.
And the results (since the logfile was last rolled over just under five hours ago) are:
[04:45:48 root]$ grep Completed mainlog | wc -l
[04:48:01 root]$ grep rejected mainlog | wc -l
[04:48:05 root]$ grep Rejected mainlog | wc -l
Which is to say, 11871 junk, 3197 of which were viruses, 107 legitimate mails.
Welcome to the internets.