What a coincidence - I saw this film last night, too.
Without going too much into specifics - yes, it does evoke Alien Nation at many points, but never in any bad way - I was intrigued and delighted by a number of elements.
First, we owe Peter Jackson a huge debt of thanks for recognizing Neill Blomkamp's talent and ability. I think he may have brought a major, major talent to the world, one that might have otherwise never broken out of his native country. You can sell me a ticket to Blomkamp's next three films right now. I think twenty years from now he'll be mentioned in the same breath as Scott, Cameron and Kubrick. Just raw talent all OVER the screen.
The story is on the one hand predictable, but continually delights by never quite moving the direction you think it's going to. One time after another, Blomkamp sets up a slightly cliched scene - which is no bad thing, providing a little audience comfort amid a very discomforting movie - and then presto-change-o, spins the situation a different direction from the one you were expecting. Very deft and clever, most times.
The main character is not just unlikeable, he alternates between being a stuffy little prick and a spineless toad. He's a Gaius Baltar who can muster more unwarranted ego. It's deep into the movie before he really shows any change.
Visually, the film is almost claustrophobic. Nearly all shots are close to 1:1 zoom, bringing the sweaty, bloody, physical struggle right into your lap. Only in a few shots do you get a wide view, and then it's back to You Are There. I don't know if this was completely intentional or a limitation of the film budget, but it's very effective.
The thumbnail description that kept running through my mind was: Cloverfield meets A Clockwork Orange.
The ending is incomplete and sets up a sequel more loudly than some films planned as trilogy-starters. IMHO, the ballsiest move would be to never make a sequel and leave this magnificent piece of work to stand on its own.
The second ballsiest move would be to throw a $200m budget at the followup and let Jackson and Blomkamp hold on to the exact same feel and presentation.
In the end, though, what impressed me was how little they bother to explain about anything. There are HUGE questions about the whys and wherefores and how-comes... and Blomkamp just sweeps them aside to tell his threaded story.
This is NOT for anyone under 15-16. Very violent, more f-bombs than any two movies I've seen in a decade (and they are not window dressing, many of them sting) and some very complex and troubling issues even for adults. Which makes me wonder - today I went back to the same theater leading a flock of short people (to see G-Force, gawdelpme and here, take my eyeballs, I won't be using them any more)... and watched a family with kids of about seven, nine and twelve traipse into District 9. Unbelievable. Have fun with the group nightmares for the rest of the month, mum and daddums...