Final Fantasy: The Spirits Within is an intriguing film. Released back in 2001, it was a bit of an unmitigated disaster for Square, costing them a lot of money. To this day, many people consider the later animated film by Square, Advent Children, to be vastly superior (although they are very different). However, whereas Advent Children was a movie largely lacking plot and character development, The Spirits Within maintains some level of story and characterization...
Easily the first thing you will notice about The Spirits Within is that, even today, it looks amazing. You can see the pores and freckles on skin, hair moves independently, the facial expressions and nuances of each character all look fluid and real. After ten years, it still remains as a testament to the capability of CGI as it continues to improve. Many other CGI movies since The Spirits Within have failed to capture the human-like qualities this movie does. I certainly don't think, either, that this is in a bad place in the Uncanny Valley.
The story, however, is not as strong. It tends to occupy the realm of "be good to Earth" that many sci-fi films do. It's passable, maybe even solid, and manages to move the characters along, but it doesn't really do anything to make this message new or original. This is compounded by the use of a villain right out of sci-fi stock. He is predictable and very typical, and it means the conflict isn't as compelling as it should be. You'll see his role and decisions from a mile away.
The rest of the cast is decent. Aki, the female scientist and protagonist, is a strong-willed, independent woman, who is, rather refreshingly, not sexualised at all. There are other typical character types here; the wizened old biologist, who is easily one of the standouts as a perfect mix of intelligence and compassion, the concerned love interest, and the comic relief. They're all mostly genre standbys.
I can forgive some of this, however, because, perhaps most surprisingly, the "acting" in this film is rather great, whether you watch it subbed or dubbed. Buoyed by the realistic animation, the voice acting feels incredibly natural and comfortable. I daresay, the "acting" in this film is substantially better than the acting of many modern day sci-fi movies. There are a lot of big name voice actors in both languages, but they all perform their characters admirably.
While the story itself may not be a home run, director Sakaguchi and company keep up a relentless pace, never letting the film get too terribly wordy and preachy. The action scenes are generally well choreographed and intense, with the "spirits" that occupy the conflict inflicting some pretty terrifying means of ending a human's life, and the story moves along at a brisk pace. While you have to deal with some wishy-washy new age mysticism regarding said "spirits," and a living Earth, it doesn't really detract from the film as a whole. Yes, the environmental message is pretty heavy-handed here, but it's incorporated well enough into the story and justified enough that it doesn't detract too much. Same with the spirits and mysticism aspect. It all makes sense as part of the scheme, and it's not just thrown in just for the sake of being there.
A quick word on the ending; I actually rather enjoyed it. It does a good job answering enough questions but leaving the future of humanity and the planet somewhat up in the air. We don't really know if anyone outside of Aki knows exactly how the Earth was "saved," which is a nice bypass of the standard, leave-some-people-alive-to-tell-the-story device.
Ultimately, I consider Spirits Away one of the most surprising animated films I have seen in awhile. While the story doesn't set the world on fire, and the nature of the conflict is a bit cliché, the strong acting, solid lead characters, and exciting pace keep the film very watchable and enjoyable. If you're looking for a true Final Fantasy film, go elsewhere, as this has pretty much nothing to do with the games. If you're looking for a generally exciting sci-fi movie, this should definitely pass the time.