Ratatouille is the kind of animated movie that mesmerizes you with its details, inventiveness, and story.
The subject matter is simply a genius idea. Rats have been used before as animated characters, but never like this. Their world is unreal, but somehow, anchored in reality. The sheer amount of details each scenes has is mind blowing. The dialogs are strong and well thought out. The scenario impeccable and won't let you go once its starts. The voice acting is superb. All of it, pure Pixar.
On an interesting note: apparently, they finally got the hang of animating wet fur, and seem pretty close on getting it for wet hair.
The only negative thing I can find is that Skinner and Anton Ego are both designed a bit too strongly on the "evil" side, which is not a habit for Pixar. However, that's very minor, and they both fit very well in the decor.
A must see.