When I found these ratings, I was not surprised at all, although I was surprised to see some reviews defending him. I can safely bet that whoever wrote those reviews were not students. Yes, maybe it's fortunate to have him as a researcher at UCLA, but these reviews are supposed to critique his teaching ability, and only that.

If a professor is strict, gives hard tests or has a bad personality, I won't knock him/her for it as long as he/she CAN TEACH and I LEARN. I've had some professors who were really difficult, but were great teachers and so I understood their subjects pretty well. Chapman's lectures are unorganized and much of them were spent on tangents and tirades demoralizing pre-med students and his personal life. He spent one entire lecture about how to write CPR's properly (just grammar)! Where's the o-chem in all this?
Add to that his crabby attitude and lack of concern for his students and you got one of the worst professors you can find. I didn't learn anything from him, except knowing what kind of professor to avoid. If he didn't have tenure, he would definitely get the axe.

"Dr. Chapman's courses are clearly not for everyone. But for the right students, they are well worth taking." Well, you're right in saying they're not for everyone, but they're also not for the "right" students who actually care about learning.