When the movie came out in 2009 I pretty much thought the same when I first saw the poster. Well that is where you are tricked.
The movie has a completely new idea about people and aliens living in a way together. It is one of the most inventive movies in the last couple of years. The mix of politics with all of the sci-fi is awesome. Also it is filmed in a rather weird way, at the first part presenting itself merely as a documentary movie. While later in the movie everything changes.
I ain't giving spoilers!Only thing I can say is that you must see it!
Here is the trailer: