It was horrible.
There, have you changed your mind? Because I can't provide any arguments for you without ruining all the closely-kept secrets of J J Abrams and crew, and then you would hate me and the movie. So take my word, don't go, and read the following. But consider yourself warned about the spoilers.
Plot Synopsis: A group of friends is throwing a going-away party for their best friend who is leaving for Japan. He fights with his love interest at the party, she leaves, and while his friends are counseling him on the fire escape disaster strikes. When everyone in Manhattan is told to evacuate (something attacked from the sea), he gets a phone call from said love interest and decides to go back and rescue her. The friends, like all good friends, go with him and are slowly picked off by the rampaging monster until noone is left. The end. (Of course, we know all of this from the videotape his best friend was making at the party and kept running all night long).
Things I will grant before I criticize:
1. I watched it with a horrible audience.
There was a row of 13-year-old boys in front of us who had been chugging Red Bull all night long, two rows of high-school girls in front of them who started groaning and complaining loudly when the color bars showed at the beginning because they didn't realize that it was part of the movie, and an overly-critical and very audible couple behind us who kept making wisecracks about how well the camera was holding up.
2. It really did make me sick, which probably colored my impression of it.
I inherited my mother's stomach. I can read for hours in the car, ride any kind of ship or boat, and eat a funnel cake and pizza right before I go on the Tilt-A-Whirl. But the sheer visual mayhem was the sort of torture you would expect Sidney Bristow to endure in a remote Kazakh prison, not something for the modern movie-going public to see in a friendly neighborhood multi-plex. Just thinking about it gives me an eyeache and makes me queasy.
3. It was impressive.
There were a ton of special effects, and the 9-11 imagery was actually some of the best stuff in the whole movie. I know there was a lot of criticism of those shots in the weeks leading up to the release, but I felt like they successfully evoked the feelings that Manhattan was under attack, and that these were people who had seen something like this before. They lent verisimilitude, actually, especially because they generally imitated those real hand-held shots of the first plane crashing into the Twin Towers. And the lack of a soundtrack was really effective, because it cut out any clues as to what would happen next. It was much less corny than it could have been.
That said, I've got three main reasons for intensely disliking this movie.
First, internal consistency. "Hud," the best friend holding the camera, is a bad photographer; at the party, he can hardly keep all of someone's face in the shot for more than 3 seconds unless he is standing perfectly still. He doesn't even pay the attention to tracking that a normal person does when they hold a video camera. And all of that would be fine except for the times when he becomes a remarkably masterful cameraman, like in the stairwell. He is suddenly able to not only fade in and out with great timing while keeping the lens directly focused on the floor numbers, but he is also able to keep it from bouncing too wildly as he climbs. Huh.
Second, scariness. Or lack thereof. The scariest part was when they didn't know what the monster was, or even what was happening. As soon as they showed the whole monster, which was very very early on, it lost any element of suspense and became simply horrible. They didn't even save some aspects of it, like the little mini-monsters dropping off it, for later in the movie. On top of that, "Hud" kept making inane comments at the tensest moments, which completely deflated the fear. I can grant that this may be part of his character, and we all know folks who would crack a joke at a funeral, but it didn't help the movie to be any scarier.
Third, bad story writing. I never felt like I really could become attached to the characters -- especially "Beth," the girl he went back for. She seemed like a selfish, whiny person at the beginning, and you almost never saw her until close to the end, but she's the whole reason this pack of twenty-something civilians is wandering a war-zone Manhattan. We only see things from one perspective (interesting on a epistemological level, but not as a movie-goer). Even the hand-held camera isn't showing us "everything Hud sees" -- it shows us "everything Hud remembers to point the camera at, which usually excludes eyes, chins, or both." Even though one girl escapes, we never find out what happens to her. We don't know what happened afterward, except that the tape was found in "the area formerly known as Central Park." There's no resolution.
All in all, a waste of money. But if you still want to see it after this, you only have 3 options: die here, die in the tunnels, or die up top. Take your pick.