I think that people can be divided into two groups: those that love Jerry Lewis movies, and those that hate them. There doesn't seem to be any middle ground.
Having said that, I am a confirmed hater of Jerry Lewis films. I find them sophomoric and excruciatingly bad, and each one has the same basic plot: Jerry Lewis' character is introduced, then he proceeds to stumble through the rest of the movie.
I'll make an exception for this film; it is the one winner out of all of Lewis' losers. The plot has been described in detail here, but it's basically the Jekyll-Hyde plot placed in a college circa 1963, where nerdy chemistry professor Jerry yearns for the love of one of his students (Stella Stevens). Lewis' transformation from Dr. Kelp to ultracool lounge lizard Buddy Love manages to be both campy and fascinating.
Great cinematography and good acting, plus a warm message and the luscious Stella Stevens make for a fine movie experience. As far as I am concerned, you can throw away all of Lewis' other movies and keep this one...it's a winner.
