Patrick Nielsen Hayden offers a brief guide to some of the best commentary on this profoundly important subject; this is a place where you really want to go and then click all the links when you get there. You'll be glad you did.
My opinion? Well, as an Aaron Sorkin fan I know not to underestimate Pluto, since Pluto doesn't know when to quit. But I figure that ultimately “planet” is not really an astronomical designation, but rather an astrological one. That gives us Mercury, Venus, Mars, Jupiter, and Saturn, plus the Sun and Moon. Neat and tidy. Don't confuse the issue with all this science!