The problem with Final Fantasy Spirits Within, was it was not a "Final Fantasy" movie...people saw it expecting it to be like the games. To have similiar ideas and theories in it. It was a completely separate entity, just had the name Final Fantasy. If it was just called "Spirits Within" people would not have bashed it as they did. The expectations were too high.
I have no problem with "games" being turned into movies, as long as they are only uses as a basis for the movie. Take Tomb Raider for example. You have the character from the game, same type of theory, but that is all. The moive itself is separate, does not have the same storyline as the game.