I feel like the bad reviews are forgetting that the movie adaptation of course is going to be different from the book and especially the fact that this is a movie for children.
It's such a beautiful movie, to the art style, character design and story. The subtle details and character development are amazing. The subtle way the robot, Ros, begins to speak less robotic and more human-like as she overrides the code throughout the movie. The tiny background art style changes in the island versus the human city.
Of course there are little parts where you go "That wouldn't work in real life!", But it's a kid movies, not a nature documentary.
The voice actors were well cast and you fall inlove with the characters.
I highly recommend going to see it, although not for very young kids. The movie is heartwarming and lovely, but a couple scenes could scare little kids :]