I was very impressed with that movie. Too often I catch a film that is a complete let down, and it will typically have a huge budget, a least a couple noteworthy actors, and gobs of special effects that are either cheesy or over the top.
And then District 9 comes along and gives us gold, and with a tiny budget.
One thing did bother me though, apart from the ending. Minor gripe but it does irk me. In the movie you see all kinds of armed troops with a variety of weapons that seem perfectly fitting given the setting in Africa. The use of bright white Beretta CX4 carbines seemed cartoony to me. Those things suck.
The prawn mech was badass though, they definitely nailed it with the end fight scene I thought.