So I'm a little late on seeing this one.  I recall there being quite a bit of hype around this movie but I'm not sure what the hype was about so I can't really say as to whether or not it lived up to it's hype so I'll just skip that part.
     At first all I could think of to describe this movie was that it was "different" and really that's still what comes to mind now.  Don't misunderstand, I enjoyed it, I think that it's just that the movie doesn't really fit into your normal category of alien films.  Normally you have either evil humans killing innocent aliens (think Avatar) or you have alien invaders coming to destroy humans (think Aliens, Independence Day, or any other alien related film), but in this case you have an alien vessel that's basically drifted to port here on earth.  They don't come off the ship but the humans go and board it only to find a great humanitarian crisis where the aliens are malnourished and in need of care.  Their technology is slightly above ours but not by leaps and bounds as is the case in most other alien movies.
     Then you end up with the social issues that would naturally develop from adding over one million members of a new species to our world.  There's rights issues, segregation, equality, etc to deal with.  The film itself does a great job of pointing out that this isn't your normal alien film because it starts in a documentary style fashion and carries that theme throughout the movie.  It's as if we're looking back after certain events have taken place and we're left with a potential sequel.  There are parts that can be a little formulaic such as the evil corporations performing experiments on the aliens in order to advance their own profit margins.  But outside of that the cinematography is well done, the story is great, and the cg work is on the mark.  
     I think what throws people off is that this movie is not your typical alien coming to earth movie.  It spotlights societies reaction to the addition of a new species of intelligent creatures to our planet.  It's not a social commentary nor is it making any political statements.  It's almost like putting the human condition with all it's strengths and weaknesses in a lab and running experiments on it.  When the film finished I wasn't quite sure what to think as there were several unanswered questions, but not so much that it ruined the film.  All in all I say it's worth a watch . . . I don't know that I'd run out and buy it or anything, but definitely something worth watching.