This movie was not at all what I was expecting. The trailer (below) is misleading, it even includes a scene that distills one of the major thematic elements, however it is not present in the movie to my recollection. I went in expecting a humans vs aliens typical movie a la Independence Day or the like. While content along these lines is certainly present (don't worry there are plenty of things exploding) - District 9 takes an interesting twist on things. I don't want to spoil the movie for anyone interested in seeing it, but the aliens are essentially refugees on Earth and this movie covers a time when humankind has decided that the aliens must move to a new refugee camp.
Since their time on Earth the aliens have gained some rights and this movie seems to be an abstraction of early rights movements / political struggles similar to what we see today or from recent history... except with Aliens.
In the end, I like the movie quite a lot - however I don't believe that the trailers really do justice to the major thematic elements that are at play in this movie.
Be forewarned: This movie is violently bloody and gruesome at many points.