This is a rather strange movie. Filmed in a street-documentary way, with lots of choppy hand-held scenes reflecting modern war footage, it is the story of an ill-advised attempt to relocate aliens from their shanty-town to concentration camps farther away from the human populace. The aliens are somewhat sullen, violent arthropods who suffered some disaster in space that left them starving and leaderless. I had to wonder how HPL would react to creatures that resemble giant, intelligent seafood. The humans call them “prawns”; it is the first sign of the disdain with which they are viewed.

The movie is set in South Africa, and obviously the alien-hate can be read as an allusion to racism, but it is not really played to excess. It is just the backdrop to an interesting drama, played at both the personal level and a world-wide level as the humans find the prawn are not all as simple-minded as they believe. Three of the prawn have been planning for twenty years to escape the prison that is earth.
In a review for SFX, Nick Setchfield remarks:

This is pure body horror in the lineage of Alien, The Thing and The Fly.

That is certainly an element. Don’t go see this if you are easily grossed out. He also has these comments which I found interesting:

Wikus van der Merwe is a nepotistically-appointed bureaucrat, a Pooterish Afrikaaner charged with overseeing the mass eviction of the alien township. Tank-top, Chuckle Brothers moustache, neat side-parting. The word dweeb is all but laser-etched on his forehead. From the moment you clock his nervous weasel features you know he’s one of those middle-management peons cinema routinely earmarks for death in a moment of grisly, crowd-pleasing black comedy.

And then there’s a moment when you realise, with sudden, amused shock, that this is the hero, that they’ve handed the movie to this man (actually, it may be the moment that he ditches the tank-top). Sharlto Copley is brilliant as Wikus, mining unsuspected reserves of heroism and furious morality beneath the nerdish exterior …

The review has some slight spoilers, which I won’t repeat, but I did think the movie needed a final scene which never comes. Director and writer Neill Blomkamp said in an interview that while Hollywood always ties up the loose ends, real life isn’t always so neat. Well, that’s why we prefer stories to real life for entertainment, Neill. Still, a thoughtprovoking and compelling movie — you’ll either love it or hate it, apparently.