A little slow to start, but...

Rating: 9/10 (5 February 2008)

Once it gets going the pacing is maintained with relentless intensity. The slowish intro is absolutely necessary in order to establish the context for the rest of the film.
To begin with, the soundtrack is exceptional, creating an involving atmosphere that draws you into every scene, and where appropriate, offering plenty of monster sized impact. It's really one of the stars of the film.
The characters are well played by a bunch of unknowns, who have all the attributes of real people, sometimes frustrating, at other times making the viewer feel truly sympathetic to their situation. (Thank god it wasn't Brad Pitt or Tom Cruise - or any of the other bunch of 'celebrity' actors that put you off going to the movies). Its well scripted too.
It is not obviously CGI heavy, though where it is used it is done very well. The general feel of the film is, well, imagine Blair Witch crossed with Alien crossed with a much darker version of Godzilla and set to a backdrop of 9-11 New York. Don't go expecting a light weight Emmerich movie, this is something much darker, and IMHO, better.
It does have a few failings, like a magic camcorder that is able to run the duration of the film on a single battery, and and at the same time provide a stellar surround soundtrack. You do need to suspend some belief, as to resolve it would probably require a move away from the first person perspective, which would change the whole feel of the film for the worse.
There is also the obvious omission during the film of any references to 9-11, though I guess one must be sensitive to such things.