When I first saw trailers for this, I wasn't sure if I was really going to like it. It looked oddly calm and possibly dull, and the animation seemed choppy. Turns out those are actually qualities. This film oozes with style and heart, and the calm and unique style is integral to that. It's actually quite fascinating in context, and not annoying like I expected. One particular scene involving a rat threatening one of the secondary characters to a beat was pure genius.
There's a crazy amount of artistic vision in the making of this film; characters never do what you'd expect, and nearly everything that happens in this movie has a twist that makes it much cooler, be it visually, auditory, conceptually or atmospherically. As for the voice acting, it is excellent and nuanced, with some great casting, and contributes very well to the whole feel of the movie.
In a sentence:
An extremely cute and simple little movie with some surprisingly smart and original artistic flourishes, and some excellent voice work.