I saw this tonight, in pretty much ideal conditions- on a blanket, at the drive in, on a nice balmy evening. It moved well, as I think everything I've seen from Abrams did, but I felt as though the emotional arc and the monster movie stuff weren't well integrated, and the monster aspects gave you just enough information to feel full of holes-
Why were the military moving it? Why did they have its ship on the same train? If it could control metal, how did they keep it contained? What was it doing with all those hanging bodies- particularly if they could all psychically contact it and have it understand they meant it no harm? Why did the military let the science teacher go with all that secure footage in the first place? What happened to the fire they set? If they were so incompetent they couldn't figure out a subterranean creature would be maybe underground, how did they catch it in the first place? Etc, etc, etc.
None of them are really questions that matter, but essentially I felt like the characterization of the monster didn't quite work- it's viewpoint wasn't clear, but we had too much information about what was going on to feel like it was purely alien.
That said, I thought the kids' performances were great, particularly the leads- the backstory with the mother was executed a little clumsily at times, but the actors managed to sell it pretty well, and the relationship between them seemed very sweet and genuine. I'll be sad if this winds up being on my top ten for the year, but it's a nicely ambitious idea for a summer blockbuster and I'd be happy to see more movies work this hard at achieving real characters and a well-realized environment before blowing shit up.