Only got round to seeing it yesterday. Much weirder than I expected, and since I haven't read the book I don't know if the underlying theme - anthropomorphic creatures who are really wild, amoral animals - is Dahl's or Anderson's. But it was gripping enough to watch and the animation is beautiful.
It also has probably the strangest, most "in" in-joke of any film I've seen. When he makes his speech, Mr Fox is listening to rousing music on his personal stereo. The music is the Grand Choral from La Nuit Americaine, Francois Truffaut's film about film-making.