Rat's are not generally nice. We don't root for rats. Ratatouille is a film with a rat at the centre. And not a stylised "cute" rat in the mould of Micky Mouse. This is a proper ratty rat, who scurries and scampers and has many hundreds of swarming friends and family. Who don't always wash their hands.
That you know all this, and see all this, and still want the best for Reny, the gourmet cook with the wrong kind of whiskers, is testement to just how good this film is. The story is good, if it does sag a bit in the middle, the characterisations and voice acting is great and the rendering is beyond reproach. Everything looks great, and the images have an effortless quality about them. One of the great things about CGI is that you can take the camera anywhere, and they do this wiith verve and aplomb.
This is an excellent film at every level. Just go see it and give thanks for Brad Bird.