Films that are turned into novels afterwards most often produce disastrous literary results.
Film adaptations, on the other hand, are a different business altogether: as these two media are so profoundly different, they rarely survive the transition from one form into the other. If you were to incorporate all the depth and detail of a novel into a film - it would be a horrible movie. If, on the other hand, you cut too much, you are left with a pitiful skeleton that doesn't have any of the stuff that made the novel so grand to begin with.
I can think of a few cases where the films were actually better than the novels that inspired them, but that was either due to
a) the film being just loosely inspired by, say, a short story[/b\ that was then VERY fleshed out by the script writer, or
b) the book being so incredibly bad that the film could actually improve it a little bit.
An example of the first would be "True Recall", based on a short story by Philip K. Dick that bore but little resemblance to the flick.
An example of the second would be "Hannibal" - which, admittedly, was a horrid movie, but trust me: the book was even worse than that.