Let me start by saying one thing: this movie is fantastic.
Upon it's release in 2002, Final Fantasy: The Spirits Within became one of the greatest box office flops ever. It's difficult to see why, except that it was a clearly a disappointment to fans of the Final Fantasy games. Never having played one, I had no preconceptions, and found the film exciting, deeply spiritual, emotional and overall, thouroughly enjoyable.
The computer generated effects are mindblowing, although they are starting to look slightly dated. The film uses "photo-realistic" CGI actors throughout, and while not entirely convincing, they come pretty close. The lead character, Dr. Aki Ross (the only fictional person ever to appear in Maxim's 100 hottest list) is genuinely impressive, both in terms of realism and character development.
The storyline is based on the popular Japenese genre of mixing technology and spirituality and, at first, can seem trivial and pretty basic- a straight through action flick. But it's when you get underneath the main plot that the film comes alive; it works on an incredibly deep level, dealing with death, renewal and hope. The film is deeply existential, as well as convincingly spiritual, and as such can be enjoyed by both kids (who will love the action) and adults alike.
For the more discerning fan, the character development of everyone but Aki is limited; some of the one liners are dry and predictable and the actors can at times be difficult to watch, due to their occasionally odd facial expressions and the fact that one (disturbingly) tends to forget they are not real people. However, this is compensated for somewhat by the soundtrack (which is somewhat mythical and generally excellent) and the action scenes.
So here you have the perfect blend: sci-fi, action, romance, spirituality and at times, humour. Overall, a great film, well worth the cost (it's pretty cheap now). Buy it and join the revolution.