The story centers on a group of M.I.T math students who along with their 'mentor' come up with a new system of "counting cards" -- if you haven't read the book and don't mind a little math in your light reading then I would definitely recommend it. Although I wouldn't recommend trying out their system, unless you don't mind the idea of getting roughed up by pit bosses.
The film was directed by Robert Luketic, who isn't exactly a household name, but is probably best known as the director of Legally Blonde. The book was adapted for the screen by Allan Loeb (Things We Lost in the Fire) and Peter Steinfeld (Analyze That). Joining Spacey and Bosworth are Laurence Fishburne in what looks like the role of a casino boss (better known as "the Heavy" in the flick) and Jim Sturgess as the newest member of the gang. 21 is set for release on March 28th, 2008.