Just got back from seeing it. I think it was great. In fact, I plan on seeing it again, maybe even more than once. It was like the Blair Witch Project, except not boring.
I kid (sort of). Blair Witch was alright.
The little spider things gave me massive Resistance flashbacks. They were an nice addition. It really adds to that "nowhere is safe" feeling, because after seeing the spiders once I forgot about them, so once the group went down the stairs into the subway I thought it was going to get boring. I'm actually kind of glad I forgot about them though, because it was such a surprise at a moment when I thought things were going to be dull.
Overall, I'm kind of ambiguous about the monster. The thing with Blair Witch is that the movie never actually showed something supernatural, but it maintained that atmosphere. That wouldn't have really worked well with Cloverfield though, because it's on such a larger scale, so I was okay with it. But I do think that maybe they shouldn't have shown the entire monster. Bits and pieces here and there, maybe even a shot of the head would've been okay, but I think it kind of ruined the "fear of the unknown" aspect of it by showing full body shots.
Oh well. It was still an awesome movie.