Overrated: Citizen Kane, Lawrence of Arabia, Citizen Kane. Oh, and Citizen Kane. I heard about how wonderful it was for years and years; when I finally saw it (in a theater no less, not on a tv screen), my first last and only impression was "Just how many minutes of my life am I never going to get back?"
Underrated: To Have and Have Not, the Carry On movies, Guns at Batasi