Blomkamp does the amazing by combing roller coaster excitement with an intelligent context that makes all the noise about something. While some of the ideas are occasionally heavy handed or on the nose, it's so exciting to see a Hollywood action film that traffics in ideas, that I'm willing to forgive the occasional 'I get it already' moment, or logic lapse, or easy coincidence, or mediocre supporting performance.
The story is simple. A million plus grotesque looking, starving and sickly aliens get stranded on Earth when they essentially run out of fuel. And while at first humans try to reach out in friendship, before long corporations are trying to make money by exploiting their technology (especially, or course, their weapons) and the aliens are herded into a 'district' and walled in. (That the film is set in South Africa is one of those choices that feels like gilding the lily - even though that is the film-makers home). And it's clear that progressing from apartheid to Nazi style medical experiments and concentration camps is the next step.
While I wish the villains were less mustache twirling (some 'banality of evil' would have been even more chilling), the effects are great, and it's a film that grew, not shrank as it went along. For the first third I thought it was OK, the second pretty good, and by the end I loved it. And it's stuck with me.