I really enjoyed it.
I totally understand the criticism of "why are they coming to a planet that's 2/3rds water when it can kill them?"
My theory on it is: they are from a planet that is completely out of food. They are starving and dying off at an alarming rate. Then, they discover a habitated planet with plenty of food. Only problem is it has an abundance of a toxic substance and the creatures on it are intelligent. But they are running out of options. So, they go down to that planet, send off a few scouting parties and wait for the right moment. Then, they make their play, knowing that many of them will be caught/killed. The ones who aren't caught? They make off with a fresh supply of meat (we are never told in the movie how many people were abducted). These aliens know that this is probably their only opportunity to do this because we'll be more prepared for them next time. But desperate times call for desperate measures. They came all that distance in their spaceships because we are the only planet with a food source around and they are starving.
It's like a bunch of teenagers doing a "snatch & grab" in a convenience store: they all filter out across the place, look nonchalant, then on a signal grab what they can and run like hell.