This movie can be criticized as not having the hipness or technical quality of anime films today, but it is the depth of the story and passion of the art that make it such a classic. I'm not a big anime fan, and this is the only anime film I've seen that I would want to watch more than once.
The story is a wonderful and surrealistic coming-of-age type allegory. Despite elements common to science fiction (man vs. machine, hero setting out to avenge his parent's death), it stays free of cliche and retains an air of realism, or true surrealism. Almost all of the characters are more memorable and unique than most main characters in other anime films.
One of the unique things about this film is the way it conveys emotion so powerfully. I can't really define what gives it this quality, but it is extremely moving, like a good symphony or vast impressionistic landscape. The only other films I can think of (at the moment anyways) that have this quality would be the Godfather films.
In conclusion, anyone who appreciates what science fiction is about should see this film. It's a rare treat.