I'm a big fan of this one. I'm still somewhat annoyed that it was encumbered with the Cloverfield burden, and I think I probably would have been less certain about the way things would go without it, but whatever, as a marketing decision I'm sure it's helped it be successful and that's cool.
Like everyone else has said, the cast is so good. John Goodman is just the best, and Mary Elizabeth Winstead deserves all of the praise for not being overshadowed by him.
I have a couple of issues with the execution of the ending, but I'm totally on board with the change of pace it presents. I think the moments when she first exits the bunker and goes through all sorts of emotions before seeing the ship in the distance, a surprisingly understated moment before what it leads to - was all extremely smart. After that I liked some of the decisions less, but not enough to ruin it for me.
Smart is the word I would use to describe most of the movie. It's nice and tight and clever, perhaps to a fault. Everything that happens in the movie, everything the characters talk about, it all has such an identifiable purpose that it starts to feel contrived, no matter how well executed. I think it's the biggest weakness of Trachtenberg's direction as a first timer (which is otherwise very strong), but I'll be very interested to see what his next project is.