Saw it last night. I thought over all it was a pretty decent movie. I mean you go in wanting to see a monster rip apart New York City, and that's what you ultimately get. I thought the acting was great too, some of the best actors I've seen in ages, they didn't go melodramatic and hokey every 3 seconds.
The only problems I really had with the movie was that the monster never took damage despite the whole movie the military pounding away at it with the highest level of weaponry they have. The monster also seemed to get around very fast and unnoticeably, you'd think that a monster the size of a sky scraper would be a little bit easier to spot sometimes in wide open areas? And I disliked how as the movie went on it kind of got a little more and more phoney, I mean a lot of the times early on it seemed quite realistic in "Here's a monster ripping through the city" and how everyone would react, but as the movie got closer to the end it just seemed to get more and more fake at so many parts which kind of bugs me since this isn't supposed to be a 'movie' but more of a documentation of real events, just loses points in my eye when the 'real events' end up looking too much like a movie or FPS (I swear this movie has FPS material written all over it). But ultimately I could ignore all that if not just for *ending spoilers*
(SPOILER)If they effing told us at the end if we won or not. It's annoying not knowing if they killed the monster or if their last resort did squat and it survived. I can stand not knowing what the monster is and where it came from so long as if I at least know how everything ended.
All in all I'd say it is worth seeing, just go in with low expectations like I did and you shouldn't be terribly disappointed.