I'm sorry, but this just drives me insane when people say this isn't science fiction. I know this comment prompted a large discussion between Flave and Shaded Dogfood, but I have a little bit more to add. This is a result of people expecting Sci Fi to be exploding space ships or prancing robots or laser guns, but the defintion of Science Fiction is speculative fiction based on science. This movie defines science fiction.
Josh Z, I disagree with your spoiler comment, there is no indication anything would be a hallucination.
As for my view on the movie: It was a stunning achievement in modern filmmaking. It has some of the most skillfully shot action scenes in the history of film. Cuaron has always had these super long, everything happening, takes in his films (even Prizoner of Azkaban) and it just shows that the man has balls. Many of the special effects during the scenes were practical and most of them are actual continious takes. I loved it.
To those of you not reading the spoiler comments, don't assume anything about this movie or what happens. It's brutal and atypical.