Cloverfield was pretty unique. I went in expecting very little, so I was pleasantly surprised by the presence of a plot with a level of character development. Of course, the best part was the visual narration. It would have been a ridiculously lame movie if it were filmed traditionally. Instead, we see all of the action through the lens of Hud's camera, which starts out as a tool to document a going away party but turns into a real-time record of a monster attack on NYC. It was a little dizzying at times, but it definitely made me feel like I was a part of the action; all surprises hit me at the same time it hit the characters. That was incredibly well done.
As usual, leave it to me to find the literary devices in anything. The action is recorded over events from a day previously filmed by two of the main characters. This seems like a completely random touch at first, but the cuts to the older recording are done in a way that accent the present action by providing backstory and irony. Naturally, I loved it.
Oh, and a little on the monster. Everyone wants to know what it looks like, etc., but those are all the wrong questions. The monster itself it just a means for the plot. Its appearance is nothing compared to how it sets the action in motion. So don't go in expecting to see Godzilla or something (oops, spoiler--the monster isn't Godzilla).
Cloverfield wasn't a gimmick--it was just a different type of movie. With that being said, however, if you don't like different then definitely don't see it. Oh, and also stay away if you get motion sickness :-P.