Jim Sturgess shines as the lead. He brings a conflict to Ben Campbell, a man who wants to do the right thing, but is seduced by the promise of easy money. He lets his emotions get the better of him and the audience benefits from his mistakes. Kate Bosworth does a decent job as the teammate/seductress, although her hairstyle left something to be desired. The rest of the team, played by Aaron Yoo, Liza Lapira, and Jacob Pitts, ably round out the cast.
Overall, 21 is not a bad film, but it does not truly excel at any aspect. Yes, it is entertaining, but it is hardly engrossing. There were moments that I liked, such as the time Ben spent with his nerd pals Miles (Josh Gad) and Cam (Sam Golzari). They helped bring out the human side and give him something other than his descent into the lavish Vegas lifestyle that seduces him.
Bottom line. In the end, it is a film that is fun while it lasts, but it is hardly great. The performances are fine, the story is interesting, but the execution seems uninspired. You could definitley do worse, but I guess it does what it sets out to do — tell a fun story that diverts you for a couple of hours.