I've only been to about two late night showings, one of them was 'The Hangover' which started at about 11:30pm, and the other was the first screening of 'Transformers: Revenge of the Fallen' at 12am. Both of them were great experiences, especially Transformers.
I tend to find that the late night showings are the ones attended by the more sensible people who actually want to watch and enjoy the movie. All the kiRAB and tween chavs are home by then because they needed to be on the last bus!