Just saw it and LOVED it. made me jump in my seat a couple of times. Will Smith is awesome as usual.
No not everything is explained but it doesn't need to be. There's plenty of inference as to what happened/happens.
Effects are great. New York looks like a wasteland. Only point of contention is the creatures jaw. When they scream the jaw opens WAY to wide.
The story is good, flashbacks give insight into what happened.
I don't really want to give away to much.
I give this HIGH marks. 8.5 out of 10. I expected more action early on. But the movie finishes in a flurry. Again I won't give it away.
Go see it for yourself. I know I plan on owning this when it comes out on DVD.