Done properly and subtly, it can be quite an effective technique. Probably harder to pull off nowadays, though, what with audiences being more aware of the trick. It's part of the reason why M. Night Shymalan disappeared up his own arse. The problem essentially is that they set a level of expectation for which the film has to be bloody good to be equal to, never mind exceed. Most films just aren't that good so they shoot themselves in the cobblers before they even get started. Metaphorically speaking of course; that would probably make a good film in itself.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'
