Most of the movie is great. It has a slow introduction, you are just starting to know the characters, party is starting to cool down, when action bangs wildly. Group of friends wondering, running, sudden attacks, a fateful decision... Unfortunately, it's hard to keep this level of intension for whole movie. Near to end, you get used to it, and what were once heroic decision and actions, now look like fool's moves. Also, things are getting a bit too unbelievable for at the end. In short, I didn't like last 20 minutes or so. I catch myself few times in wandering around in my head instead of really following the movie.
But as I said, most of the movie is really good. Movie easily succeeds in creating a atmosphere of mystery in the first part. Later, as we learn more about the threat, it looks very convincing. I must say that aliens are done very well. But after the part in military base, things went down for me.
Ending is ambitiously imagined and special, although it failed in making all the emotional impact in me. But I presume that lots of people will like. Actually, lots of people did like it, since this was one of hits in 2008.
"Cloverfield" is a good mystery/SF movie, although with potentially weaker ending. Nevertheless, it is a good evening watch, with much fun and action, for SF fans as much as for general audience.