Genius. I loved this movie. Not because it was the best movie in the world but because the entire experience was something new and refreshing. You have to watch it in theaters, there's no substitute. Sure, the shaky camera work makes people dizzy, people start throwing up, whatever. If you can handle watching your cousin's amateur wedding video, you can handle this. Cloverfield has been (accurately) compared with The Blair Witch Project but it's much better in concept and execution.
Part of the genius of the movie is turning monster movie convention -- something that's been around forever -- neatly around. Instead of creating Godzilla Part 30 or some such derivative, director Matt Reeves and long time friend and partner J.J. Abrams (co-creators of Felicity) went smaller instead of bigger; focusing on the little people.
It's impossible to watch this movie without thinking about 9/11 -- the roiling dust cloud from collapsing buildings, the shaky panic shots -- but I liked that it wasn't afraid to visually reference the event. Overall, it's just a clever and engrossing film. I think it's terribly brave of Reeves, Abrams, and Paramount to put out a movie -- with some significant marketing push too -- which will automatically be discounted by people who can't get used to the handheld cameras.
Don't believe the anti-hype, go watch it in a big theater right away before it disappears.