someone make a movie about this

I just had an interesting thought while I was 'listening' to a conversation some people were having. Okay, you caught me: they were having the 'conversation' on Facebook, and I wasn't listening so much as I was reading their comments about a status update, but whatever. The point is this: I realized can't recall having heard anyone say they watched a movie and were disappointed by the book, but I frequently hear people say they read a book and then were disappointed by the movie. I have a number of theories.

1. Books are better than movies for various reasons (imagination used, time to develop plot and characters, better writing, etc.)
2. Alternatively: order effects are in play. People usually read books first, then watch the movies. But what if order matters? What would happen if you watched a movie and then read the book? Then you'd always be stuck with what's-his-face as the main character, and you don't even like what's-his-face.
3. Alternatively: people like to throw it in your face that they read the book, you didn't, and they are smarter and all-around better than you.

I'm sure there are lots of anecdotes out there. Someone's going to have to do a systematic study to get this figured out. Not it.