I saw this last night, and loved it. The camera-work was hard to get your head around to start with but once it all kicked off it worked absolutely perfectly. They had the mix of 'cinematic' shots (i.e. shots slightly less realistic but allowed you to see what was happening, without these you'd just be watching the floor all the time

), and 'realistic' shots (i.e. of the floor)
exactly right.
For those questioning the lack of story. Well, i think the whole idea is that you were watching 'their' story. They didn't know what the heck was happening, why should you? It made a great change to simply know what they knew, and nothing else. And of course there was a beginning, middle and end. The setup was the party, finding out about the relationship between Rob and Beth etc. And there was more than a 'middle' with all the different stuff happening (subway, mall etc.).
I thought the ending was brilliant. When they crashed from the helicopter i thought it had gone all soppy on us and got a bit annoyed. But when the actual ending happened it was just perfect. Realistic.. satisfying.. just perfect. I
love the fact that you can kind of make up your on ending, or try to figure out what happened to the monster. Funnily enough, i think that's the point.
All in all, loved it. I went with 3 friends and every one of us came out of the cinema grinning like gibbons. It's a great film, and a refreshing change from the typical monster movie.
EDIT: And how good was the sound?!