It's difficult to say much about Cloverfield without spoiling the plot or raising inaccurate expectations about the movie, so suffice to say that it is absolutely brilliant and you must see it at your first opportunity.
As those who've seen the original Japanese version of Godzilla (Gojira) know, Godzilla was less about a radioactive dinosaur than it was about the Japanese experience with World War II, so hopefully you'll understand me when I say that Cloverfield is a giant monster movie about September 11.
The whole thing is shot on handicam from the point of view of a civilian witness to the proceedings and comes across a little like a much cleverer daikaiju-themed Blair Witch Project. That's probably going to turn a lot of people off and cost the film some mainstream appeal, which is a shame, because the handicam gimmick is handled superbly, used to clever effect, and forms an absolutely integral part of the movie's tone. Director Matt Reeves hasn't previously done much of note, but he ably distinguishes himself here.
This is likely to be one of the better films of the year so grab some friends with some taste in cinema and catch this one as soon as possible.