District 9 started off in a documentary/news style, which made it appear more 'real'. The film took its time building up the kind of man that Wikus was, and we the audience are better for it. From start to finish, there is a real transition in his character development, which is something that's generally lacking in films (not just science fiction films).
What sets District 9 from other alien films is that it avoids sci-fi techspeak. Yes, there are some great CGI effects, and we see all manner of explosions. But the way the story is delivered, it was social commentary disguised as a sci-fi film. It reminds us that we're all suspicious and prejudiced against things we don't understand.
This film made me feel the same way as The Shawshank Redemption. Ultimately, it's a film about hope. A film that shows how cruel people are (or can be), and how (despite all that) the protagonist is shown not to have acted in vain.
If you like films that aren't completely predictable and even a little leftfield form the norm, you won't be disappointed with District 9. Yes, certainly elements of the film are predictable (as with any film), but ultimately I doubt many viewers can actually 'see' where the film is going after the first 30 minutes.
~ squodge ~