Longer, spoilerised review below.
Anyone with any knowledge of TV sci-fi will already have made the obvious link regarding District 9, so let's start with that: yes, this is Alien Nation with gore and swearing. In fact, the film is so reminiscent of the 1989-90 TV show that assuming the Prawns have a similar relationship to their mothership as the Tectonese did with theirs is the only way the ending makes any fucking sense at all. In fact, signing off with no explanation of why the prawns ended up on Earth when apparently they could have just flown home is symptomatic of how schizophrenic the film is. The drip-feeding of information about the Prawns through documentary footage and TV interviews is exceptionally clever, forcing us to form our own opinions of the aliens (especially nice was the entomologist telling the viewers the Prawns were a worker caste; was that actually true, or just a classic attempt to, forgive the term, "dehumanise" the Other), and if the film had kept that up throughout any confusion and lingering questions would be entirely to be expected. Ten or so minutes in, though, we break from the footage approach and watch three Prawns discussing their plan to escape the planet by draining their own technology of every drop of the precious liquid MacGuffin it contains. If you can show that, then why not explain why the mothership lay dormant with the Prawns trapped inside, only for Christopher to easily reactivate it two decades later?
As I said, much of this can be bypassed by assuming an Alien Nation parallel, that the Prawns were slaves trapped in the hold, and it took Christopher twenty years to scavenge enough tech to be able to return to the ship and actually get in the driving seat (you can also extrapolate from the entomologist's contribution that Chris might be a Prawn "queen", and thus smarter than everyone else), but that doesn't change the weirdness of combining massive gaps in information with (frankly very clunky) exposition, all delivered by subtitles. It doesn't particularly work.
There are a lot of other questions thrown up by the film. How come the world's second biggest weapons manufacturer hire troops that can defeat a single administrator with only one fully functional arm? Why would a fuel source also transform humans into Prawns? What possessed the aliens to create a magnet that works on lead and pigs?
Mainly, though, the question that kept cropping up in my mind was: what is this film trying to say? Is it about the viciousness and nonsensicalness of racism? If that's true, making the Prawns pretty stupid seems like an odd choice, to say nothing of the dark irony (or unfortunate idiocy) of making almost all your black characters machete-wielding flesh-eaters (though as Gooder pointed out, almost every white character spends the entire film trying to shoot everybody they can). Of course, it's hard to see the film as anything else? The instant Wikus begins to become a hybrid, his colleagues and family (well, family by marriage) immediately treat his as no better than a Prawn. DO YOU SEE HOW FOOLISH RACISM IS? the film bellows, as Wikus' former colleagues test his nerve endings by drilling into his arm. Of course, some of the commentary is more subtle, so I guess they were just trying to cover a spectrum, for which I can hardly blame them.
I sound like I didn't enjoy the film, but I did. A great deal, in fact. The central idea is pretty interesting, it was a brave move to set it in South Africa, and there are an awful lot of brilliant moments. Sharlto Coply is great as Wikus, managing to act like a total bastard throughout and yet still engender sympathy (this may have something to do with him reminding me so much of Murray from Flight of the Conchords). It's just hard to watch without focusing on some fairly obvious structural issues, that could have been fairly simply ironed out and turned an already pretty good film into a genuinely excellent one.
On the other hand, though: prawn-shaped mecha. You're going to have to think long and hard to come up with a more awesome idea than that.