First of all, do not watch the movie without having Karas: The Revelation, too. It basically ends abruptly and Revelation starts abruptly at the same point in the story.
The film is a Japanese anime, but in a style that is not often met. The characters are designed in an original way and the usual great Japanese animation is complimented with 3D-like, very fast, action sequences. And, before you say it out loud, it looks good, not like the usual 3D crap, and it blends very well, without that feeling of changing art style.
The plot is a bit weird and not easily grasped and felt, however once you get it, it's OK. Every city is alive and it creates a creature that looks like a punkish little girl to blend with a human soul and create a Karas, which is raven in Japanese, not a golden fish. The Karas is the city protector, both in physical and spiritual senses. Also, the city is full of demons that normal people cannot see, but that are essential to the functioning of the city. The demons do feed off human blood, but usually in a non violent way, and they gain strength from the human belief in them.
Now, the bad guy comes, a disgruntled former Karas, bent on destroying all humans, which he sees as spoilers of the city. The demons can't do much about it, since people have stopped believing in them and so are weak to the point of dissolution. The bad Karas is merging demons with machines to create monsters that eat humans and obey his commands.
The premise is pretty fun, the characters are also interesting, and the conflicts, while being pretty obvious and a bit childish, are also with enough complexity to add to the film. Where Karas chines is the action sequences, which are great in term of design and scale and animation. You often get these slow motion scenes where humans just seem to freeze in time, while supernatural creatures take long conversations and battle at ridiculous speeds and power levels.
Bottom line: not as complex as many anime that I've seen, but with great animation. It's like a Japanese blockbuster anime movie: simple plot, great special effects.