First off, I really like Babylon Babies
by Maurice Dantec; it's a complex tale of social engineering and evolution, both racial and individual. Toorop, the hero of the tale, is a philosophical warrior who has seen too many conflicts and not enough resolutions.
Toorop is also around fifty years of age, Vin Diesel isn't - the beginning of the films' divergence from the book! The film tries to boil down the complexities of the book into a more linear action film, but it doesn't work. I suppose Christophe Gans might have made a better director and added some French flair to the proceedings, but it's hard to see any film really doing justice to Dantec's story.
So, Babylon AD is a fun action film, but it leaves out all the questions Dantec poses, opting instead for louder gunfights, and replaces Toorop's anguished musings with Mr Diesel's sometimes incomprehensible drawl. Even Michelle Yeoh spends most of her screen time looking slightly bemused, whether because of the script or a feeling she was in the wrong film... The other players in the story fulfil the necessary roles, but there isn't anything new or shiny to raise this above the other sci-fi actioners that unfortunately take the place of decent science fiction films.