What kinds of spam are effective?

From Alex Mindlin’s “Seems Somebody Is Clicking on That Spam” (The New York Times: 3 July 2006):

Spam messages promoting pornography are 280 times as effective in getting recipients to click on them as messages advertising pharmacy drugs, which are the next most effective type of spam.

The third most successful variety is spam advertising Rolex watches, 0.0075 percent of which get clicked on, according to an analysis by CipherTrust, a large manufacturer of devices that protect networks from spam and viruses.