This movie is great on many levels. First it works as a really good sci-fi/action movie. But there is also a message, intentional or not, and it raises questions. But it's not being put on the foreground. Yet you clearly see the references. And it still works today.
The mix of documentary and cinematic style adds a certain level of realism, as much as this is possible with an alien story, but you are much more drawn into it because of that. The story itself is interesting. You don't have aliens coming to conquer or destroy mankind. They are pretty much a sorry bunch with little hope of existence. I like that angle.
Putting this not into the hollywood realm and not placing it in America also just adds to the positive attributes this whole movie has.
I am on the fence if I would want a sequel to be honest. If that would mean it gets the hollywood treatment I'd refuse. I mean - look at the Terminator franchise for a reference.
In any case, after re-watching this movie I raise my rating from "8" to "9" because I really think it is that good. Unfortunately, from where I stand, Blomkamp hasn't even come close to repeating with any of his other movies following _District 9_.