Well, I really enjoyed the Felidae-movie. Definitely not for children (still less than Watership Down).
After reading the book, I can say: Except for two dream sequences, nearly everything was in the movie. Even the dialogues has been inherited virtually one-on-one (at least in German).
It has a great story (that's not so predictable). It has a great style. And it's dark and brutal (and yes, for a animated movie, that could be an asset).