I just finished the movie and loved it! It certainly had flaws but overall a great flick. One of the things that made it so great is I had no idea what it was about and no idea where it was going - I had only heard something about "a monster being brought to a planet" so it was a good ride for me.
Definitely not for everyone as many people would prefer to see a cop run around town solving a generic murder while his family is, of course, threatened/kidnapped/killed. Basically the kind of movie I don't waste my time watching.
I would put this movie in the rare category with Pitch Black and The Host (putting it above The Host and below Pitch Black) I wish there were more like it.