Though a sizable part of the depth of District 9 seems to have been taken from an earlier film called Alien Nation, it doesn't really make a difference. What makes District 9 really good is that it's a genuinely thoughtful movie (a rarity in modern hollywood), making commentary upon human society, and the way we treat those in our in group, and those in our out group.
In it, the prawns serve as a metaphor for many of the refugee groups which exist in our world today, and which often end up in situations quite similar to the situation the prawns were forced into. By examining the situation that humans force the prawns into, we examine ourselves.
Take the fact that it was a genuinely deep film, and add to that the fact that it really was an enjoyable watch, with good solid action and an involving plot, and then you'll understand why I liked it. A movie that's both thoughtful and entertaining is a rarity, and District 9 was a gem.