I thought Cloverfield was excellent, it did exactly what it set out to do, showed a monster extravaganza from the perspective of all the little screaming ants you always see running away in all those movies. You know, the ones we always laugh at when they stomped by Godzilla's foot or eaten by Mothra or...you know what I mean. Basically JJ wanted to give us a whole different perspective, and for me that was very refreshing. I think it was time we got to see this kind of story.
It made me claustrophobic and freaked me out. I especially loved the sequence where they were trying to cross from one upright apartment building to another that was toppled sideways leaning into it. The chase in the subway tunnel made me jump out of my chair as well. I thought the acting was pretty good, it made me think I was really watching a bunch of American yuppies having a self-indulgent party in New York before their entire world got turned upside down. I found that they were acting as real people, but some people misconstrued this as bad acting because, let's face it, the average joe is just not cut out to be an actor!
Based on reviews I had been reading since it came out, I thought I might be disappointed but it exceeded all my expectations.
One tip for those wondering what the thing was, and where it came from: