Over the last few years I've done volunteer spam analysis for a number of groups.   As part of that work I've put together a report on individual spam categories at http://www.stearns.org/spamreport/spamreport.html .  It deliberately avoids conclusions, but provides the raw data for you to draw your own.  For example, the summary line near the end might answer the question, "Has the CAN-SPAM act - which went into effect January 1st, 2004 - measurably reduced the amount of spam we see in our inboxes?"
    -- William Stearns