I had a coworker once that argued that point with me. She HATED the third movie because it changed the book so much, while I loved the third movie but had not read the book. She stated that if I never read the book, I had no right to criticize the movie. A bullshit point, to which I told her that I have every right to criticize it as a movie, but no right to criticize it as an adaptation. I then pointed out to her that by her logic, her lack of comic book reading left her no place to criticize any comic book film.
The tonal shift probably turned a lot of fans off and did not allow them to see it as a coherant movie, unlike the first two which just seemed like disconnected scenes strung together culminating in a mostly boring climax. The further in time we get from the actual release the better it seems to be received even among hardcore fans. I don't think anyone will say at this point that its a bad movie.