Good effect, minimal load
Blacklisting is never better than poorest data source used
We use Bob Beck's traplist (greytrapping generated - as far as we know no false positives) at http://www.openbsd.org/spamd/traplist.gz, as well as ix.de's Nixspam (see your spamd.conf) plus the BSDLY.net list.
Prime example: Steve Williams' October 20th, 2006 message to the OpenBSD-misc mailing list - got rid of more than 95% of spam using spamd in pure greylisting mode
Greytrapping is fun and effective: See bsdly.blogspot.com