I did see it, as I live about a mile and a half from two arthouse theaters (ah, the joys of living downtown in a big city). If you knew nothing about Diane Arbus, it was a mildly entertaining fiction. The film was hampered in showing anything of her actual work because her sister(daughter?) who controls the estate won't let anyone get anywhere near Diane's work without 100% approval and control by her. And since she didn't like the book on which the movie was based, there was no way the movie was going to get access to Diane's work.
Frankly, unless you just want some entertainment, and enjoy watching Nicole Kidman and Robert Downey chew the scenery, then give it a miss. Oh, and you get to see some cool cameras onscreen. Diane's Rolleiflex, and a Sinar Norma with a 14" Commercial Ektar on it, for certain.