I had intended to read the book for a while before the movie was announced, the movie coming out just gave me a deadline to get it done.
I still haven't seen the movie, but I can say the book wasn't quite was I was expecting. It was an entertaining read; the story was intriguing, if only for the setting; it was written well enough and some characters were interesting. Though it is clearly aimed at girls/women with fairly strong themes of female independence and empowerment. I'm cool with that though, since I don't like women to be all weak and submissive and shit.
Over all I'd say it's worth reading, or just fuck it and read the very similar, and undoubtedly more violent and less female focused, Battle Royale
by Koushun Takami. Or the manga adaptation