In my opinion I do not think this is a 'groundbreaking' film by any means. I actually didn't even like the first half an hour, I felt like I had seen it all before and I didn't particulary like any of the characters. I was really expecting more due to the high praise, (and I generally like Joss Whedon's work) and I'm not normally swayed by the critics and magazines, but everywhere this seemed to recieving 5 stars and a lot of hype. So I had to see for myself.
Overall, slightly disappointed, but saying that, after that initial first half an hour, the film started to improve and I found it started to hold my attention. It did turn out to be different in a few ways to the standard film of these genres, and there's some parts where you find yourself saying 'This is pretty insane!' but simply because it seems to start coming out of nowhere and there's so much going on toward the end. It's entertaining I will give it that, and I would probably watch it again but I personally don't think there was anything scary about it and I don't fully understand why it recieved so much hype. But it's good fun and I think a lot of horror film fans will enjoy it.