One of Rob's movie rules is "The better the build up to the monster, the bigger the let down when you actually get to see it". Cloverfield doesn't break this rule. The look of the film itself is excellent. If a giant monster did attack New York, and the only document left was hand-held video footage from someone underneath it, it would probably look a lot like Cloverfield. Except for the monsters, which had the air of poorly rendered video game characters. They even had little monsterettes which looked like they were there just for the underground scenes. For my money it would would have been better if you never actually saw the monsters. Now that would have been scary.
Films have been made using camcorders before but they don't usually centre all the action entirely around camcorder footage shot by one of the participants. All the camcorder owner cliches are there, from not knowing how to use the camera to leaving it switched on by mistake.
The action starts at a party packed full of "Beautiful People" (tm). To be honest, this goes on a bit, setting up a romantic sub-plot. However, once the monsters turn up things happen very quickly indeed. And one by one the Beautiful People are picked off. The acting from the case of unknowns is very good, although it must have been very hard work for the young cast staying scared all the time. Perhaps the reason there are no older, more unfit, characters is that they would not be able to survive the making of the film.
If you like monster movies, you should see Cloverfield. Some of the scenes are excellent, and there are some really scary moments. But I don't think it will keep you awake for long at night. Not if you've played Doom or Quake.