The Spirits Within has excellent visuals and good quality audio, music and voice acting. I also like the bittersweet ending of the movie. The problem is that the film is pretty much a normal sci-fi action movie, and the only real thing that makes it stand out are the CGI graphics. It doesn't really have that memorable characters, interesting species, fun action scenes or anything which makes movies such as Star Wars stand out. This is what prevents it from being an excellent movie for an average viewer.

As for Final Fantasy fans, the movie doesn't really have an FF feel because there are hardly any FF elements in the movie. There are a few FF references, but they're quite subtle, such as Aki Ross wearing a Chocobo shirt in one scene (IIRC), the similarities between Gaia and Lifestream, General Hein and Rufus are somewhat similar characters and I can also see a bit of similarities between Zeus Cannon and Sister Ray.