Ending - Pass or Fail

So, we were wondering something. If a movie such as this has an ambiguous ending as to leave the viewer confused or clueless, has the movie failed completely if the writer/director had intended for the end to be understood? I know that the argument employed is often that the viewer should decide for themselves the meaning, but often I find this is a complete cop out. There are just some movies where it is clear the director just dropped the ball and was not competent in getting the message across. Bigger question then of course, how can we ever know?