Let's see how much I can say without having spoilers. I'll put any spoilers in my first comment.
The best thing about the movie was the monsters. The worst thing was that the people weren't very interesting.
The big monsters were exceedingly fine--scary, alien, and very smooth CGI. The close view of a monster's face changing near the end of the movie had me thinking that I don't want to see that in my dreams, and I think that's the first time I've had that reaction to a movie. Their noises were also satisfactorily scary.
The little crab/spider/scorpion things were good, but not extraordinary.
The main character was an unusual choice -- he's got much more courage than most, but he isn't an athlete, and certainly not a CGI superhero. If only he could act better.
The failures of realism I noticed were that apparently New York has no children or old people, and no handguns. That last would have been useless against the bad monsters, but the little (maybe a yard across) crab/spider/scorpions would have had a much rougher time of it.
As you may have heard, the movie is mostly (entirely?) shown as hand-held camera video. I handled the unstable point of view well enough while I was watching the movie, but felt a little queasy afterwards. I'd say that anyone who was ok with Waking Life should be able to manage the camera work easily.
Emotionally, I feel as though it's about bewildering, frightening time right after 9/11.