This movie is based on a japanese anime of the same name.
The story takes place in a world with an alternate history in a post-apocalyptic scenery.
You will fall in love with the stylised CG elements. The visual inspirations of different styles got mixed and a wonderful new style was created. For example steampunk elements, the 30's and other retro influences, and the grading adds a special atmosphere to those impressive pictures.
The awesome fight- and battlescenes round up this unique visual experience while the music is perfectly placed and fits to the scenes and the atmosphere.
But if you think this movie is just all about the visual thing, then you're wrong, because the story itself is also great and is one of the important things of the whole movie. It combines different aspects of happenings and feelings with a deeper meaning and leaves you thinking after the movie is over.
My opinion? You really have to watch it. =)