This movie got pretty dismal reviews (40% on the Tomatometer) and a lackluster 6.8 IMDB rating. But I liked it quite a bit. You probably know the sci-fi premise already, but if you don't you won't get it here: just get it and watch. It's kind of fun figuring out what's really going on at the THX1138-style beginning.
About 45 minutes into the movie, though, it's completely clear to everyone what's going on, and the movie turns from what might have been Thoughtful Social Commentary into Big Loud Dumb Fun Action Thriller. And, frankly, that's probably for the best. Long stretches where the dialog is mainly: Run! Get down! They're over there! This way! Take 'em out! Aieee! But it's still pretty good, even then, because the movie has taken its time about making the characters believable and sympathetic.
The actors are top-notch, better than the movie really calls for: Ewan MacGregor, Scarlett Johannson (very easy to watch), Djimon Hounsou, Sean Bean at his slimiest, and the always fantastic Steve Buscemi in a brief but important role. (Also Shawnee Smith, previously seen in Saw.)