I have to re-post this article from College Humor about a specific category of movies:

“Some movies are good, some are bad, some are just ok, and a very select few start out good, get you on board, then slowwwwwly get worse and worse until you finally stop caring about them.”

It’s almost perfect.

http://www.collegehumor.com/post/6894864/8-movies-that-turn-to-shit-after-20-minutes