How valid is the JEDP theory?

The [JEDP theory](http://www.theopedia.com/JEDP_theory) basically states that the first five books of the bible (the Pentatauch) were not written by a single person, but rather by four different people. I'm trying to understand if this theory is even remotely valid or if it is complete nonsense. Specifically (to avoid this being closed as argumentative), what are the most reasonable supporting arguments for this theory? Also, what are the major criticisms against this theory?