Nathan Newman points out why Al Sharpton shouldn't be dismissed as simply "a race-mongering cretin." Anyone who underestimates his appeal to African-American voters does so at his own peril. Sharpton is a race-mongerer, but he is hardly a cretin.
Of course, since he likely won't "play by the rules", his viability as a candidate will be only as a novelty act. Beyond that, it's tough to take anyone seriously as a Presidential threat when he only attracts 75 people to the rally that launched his campaign. It's a long, lonely road, Al; count on your bandwagon being empty for a very long time.