This is a movie you should watch at least twice. Not even because you will be dying to watch it again immidiately, but because you'll need to prepare for what is to come.
The filmmakers did their best effort not to reveal at first what kind of creature we're dealing with. So you start watching a very creepy beginning, in all daylight, not knowing what's chasing the siblings. It has a lot of suspence and in the first half an hour, it will creep you out. At this point you'll think you're dealing with a murderer, but then, surprise, it suddely turns out to be a monster!
That was the filmmakers' surprise. but i don't think that was the right appraoch. After being creeped out for half the movie, it is just a terrible dissapointment to see the monsters face, and it will ruin the rest of the movie.
Then watch it again, and you'll see: if you know you're about to watch a monster movie, and you know what the monster looks like, you can value the rest of the atmosphere, and will notice there is still a lot of suspence left.
I think this is one of the best horror movies made lately, and evrybody should give it a chance.