So I just got back from the midnight showing of cloverfield... It was terrific. The handheld cam works surprisingly well, and it doesn't seem corny or over done. I was impressed because pulling off this sort of gimmick isn't easy.
The plot is obviously a rehash of most monster flicks, but the suspense is what really drives you along. One of the reviewers on RT said it was more like an interactive ride at say disney/universal than a movie and I have to say this sums it up fairly accurately. The beginning sets up your characters and from then on it's almost nonstop action.
The tone is mostly dark, although you'll find yourself laughing at the camera operator character throughout. While it's a good diversion it's sometimes odd that something tragic may happen and right after he's off joking again. It's nervous humor, but it doesn't always feel appropriate.
A lot of people may have a problem with the ending, but I feel it fits perfectly with the movie. If you really think about it there would be no logical alternative.
All in all a fantastic movie going experience. Definitely go see it in theaters. The sound (which fortunately isn't handicam quality) adds to the first person experience.
Also, there aren't too many spoilers--it's a fairly straight forward movie, retrospectively, but it's very suspenseful despite. With a little suspension of disbelief you should enjoy yourself greatly.