I'll try not and spoil anything which hasn't already been in the commercials and trailers.
The movie is bested described as a cross between E.T. and Cloverfield, and the lens flares from the last Star Trek thrown in.
Normally I worry when the movie main characters are kids, but for the most part the kid actors were great. Of the core 6 kids, 4 of the were fully developed characters, 2 of them were basically set decorations and could have been left out and no one would have missed them.
The movie takes place in 1979, and focuses on a bunch of 12-14 year olds as they film a zombie movie on a super 8 file. As I would have been 13 in 1979, this movie really hit home. They did a very good job of making me feel it was actually 1979.
The train accident near the beginning of the movie was great, it blew me away. Not sure how the kids survived, but you wouldn't have had a movie if they died.
You don't get to see the "monster" until probably 2/3 of the way through the film, the way they handled it until then was very much the same way they handled the "smoke monster" in the early seasons of Lost. You see the damage it is causing, but you never quite see what it causing it. The special effects on the "monster" were very good, especially in the scenes were it is not zooming around and you actually get to look at it.
The really only bad thing I can say about the movie it they used way to many Lens Flares. Not sure what the point in all of them was, but it has become a "signature" of JJ Abrams films.
I would not take small children to this, as there are points in the movie which will scare them. My guess is 8+ should be ok, but it will scare them at points.
One nice tough is during the credits, you actually get to see the super-8 movie the kids were filming during the movie.
I would give this 4 out of 4 stars, it is a movie I plan on seeing again.