I doubt that anyone does not yet know what District 9 is about, but you can't discuss a movie without even briefly outlining its premise, so here it is. After an alien ship "parks" itself in the sky over Johannesburg, humans visit the ship and find myriads of very sick aliens, who look like two-legged, upright, larger-than-human shrimp. They are transported to Earth and placed in a ghetto, called District 9, where they live in slums. It's not clear what happened to their mothership, but apparently it can't go back to its home planet. The aliens are nicknamed "prawns", and are loathed and feared by most Johannesburg residents. 20 years later, by popular demand, the government has built a camp for the aliens in a new location, where they would be out of humans' sight; the task at hand is to relocate all 1.5 million of "prawns" to the new camp. Since the government is trying to preserve an appearance of legitimacy in dealing with the aliens, it sends an official to serve eviction notices to them. The official, Vikas, goes into the field wide-eyed and enthusiastic, but soon his life takes a very disturbing turn.
Perhaps unintentionally, this movie blended the tropes of two movies I saw when I was of an impressionable age -- "The Fly" and "Enemy Mine". I had not seen many American science fiction movies in my teens, since at the time and place I grew up they were rare enough. So these two movies were my first acquaintance with certain SFnal tropes -- and I won't say which ones, because that would be too much of a spoiler. Aside from transporting me back to the days of yore, "District 9" has thoughtfulness and freshness that those two movies did not have. Instead of a Hollywood-style superhero, the protagonist is a buffoonish bureaucrat. Almost to the very end, even as he engages in some heroism, he remains selfish and smallminded, and clings to a belief that life can go back to the way it was. His faults were no less a contribution to the movie's realism than the up-close shots of the slums and of aliens rummaging through filth in search of food. A more conventional narrative would have such a character die in his final act of redemption (most Hollywood villains die when (if) they finally see the light); instead, the ending here is ambiguous. He ends up in a state that could be seen as hopeful, or as a fate worse than death. It is similarly not clear what will become of the aliens trapped on Earth; have they been abandoned, or is help -- or revenge -- on the way? However, this is not the case where open-ended plot suggests the author has not thought it through. Instead it leaves you with a sense of possibility. A possibility of a sequel, perhaps? :-)
Oh, and in case I'm not being clear -- I liked it. While I don't watch many movies, District 9 is one of the best SF movies in my recent memory.