Cloverfield is a first person "docudrama" (meaning, it's like Blair Witch) starring some relative unknowns (I want to save you all the time -- Marlena was played by Lizzy Caplan who was Janis Ian in Mean Girls...I couldn't place her for the life of me) and directed by Matt Reeves. A big monster (and a bunch of little monsters that fall off of the big monster) attacks New York City and is taken up from the view point of four people as they struggle to survive and reach their friend who lives in the middle of Manhattan.
The effects were great (what you can see of them, anyway) and the monsters look great (no Gojira here), but the camera man was really annoying as a character, and it's the Blair Witch sickness all over again. Other than that stuff, and as a whole, it was actually really good and really needs to be seen on a big screen so that when the monster is in view, you can actually see it.
Also, if I were a Trekkie, I would be over the moon -- the teaser for the prequel is really quite good.
One last thing: is it just me or did some of the sets seem strangely small? I've read that Matt Reeves had never done anything like this before, had never directed anything so VFX intensive, and it kind of felt like that in certain scenes.