I went in expecting a big dumb monster movie. I didn't expect it to actually be, you know, scary. I was in for a big surprise. That movie is intense! It's sort of like how all those kids were dropped off to see a matinee of Night of the Living Dead by their parents, thinking it was just a harmless spooky movie, only to come home shellshocked and shaken*. The premise is basically "Godzilla from the POV of the poor bastards getting stepped on", but Manhattan has never felt more like the Nostromo from Alien. As I walked home from the BART station, I still felt claustrophobic, despite being out in the open. When the huge creature is rampaging through town, tossing rubble and crushing everything in its path, being able to run is no protection—you're just as trapped as if you're hiding in a cardboard box.
Speaking of which, it really hits on all of the big fears: claustrophobia, acrophobia, arachnophobia, agoraphobia (big time! it's practically the whole movie), fear of the dark, and a touch of body horror. Large things crushing you. Small, fast things clawing and biting you. Even gephyrophobia and aviophobia get in there. If you suffer from any of these fears, you will probably end the movie in the fetal position.
A lot of critics have pointed out that the movie has a shaky camera for pretty much the entire running time. It's true, and if you find that shaky cameras in movies make you motion sick you should probably sit this one out. I found it irritating in the beginning, when it's mostly just people talking, but when the main action starts (just when the shakily filmed conversations have really worn out their welcome), you don't even notice the shakiness, and the "verité" approach just works. You feel like you're there, looking at what the cameraman is looking at, seeing only what he can see (and, more importantly, not seeing what he can't).
The dialogue was a bit whedonesque at times ("Another something. Also terrible."), but it didn't detract.
What the critics haven't pointed out is that this is one feel-bad movie. It's not some "cozy catastrophe" romp where the main characters get to play hero while the extras are dying off around them and Heroic Impulse + Willpower = Victory. If there's a point to the movie, it's that just because something is the morally right, heroic thing to do, that doesn't mean it isn't also a really stupid thing to do**. It may even be the wrong thing to do***. This is not the usual Hollywood party line!
I thought it was great.
*The original Night of the Living Dead was astonishingly scary, considering the effects consisted of Bosco and greasepaint, and, like Cloverfield, had a really disturbingly cynical worldview.
** And not just in the "so crazy it just might work!" way, where decisions that defy self-preservation end up working just by virtue of their heroic purpose.
*** Think about the decision to go back for Beth and the condition she was in, and tell me more of them wouldn't have survived and the ones that didn't wouldn't have died in such horrible ways.
P.S. Did anybody else who watched the movie also notice that the ending theme was a total homage to the classic Godzilla score?