It was pretty good; the way it was filmed was very effective considering the movie's setting and storyline. However, anyone who didn't follow the promotional websites for the movie will probably be scratching their heads and wondering where in the hell the monster actually came from. I spotted a single visual clue, but as I said before, if you didn't follow the viral marketing sites, you would probably disregard it. I thought this was both:
A) Quite clever; and
B) Quite stupid.
Polar opposite opinions, I know. On one hand, it, well, rewards those who did their homework before hand with... with... the feeling that they maybe probably perhaps knew where the monster came from. I have a basic theory, but it doesn't make sense if I try to explain it.
However, on the other hand, it gives the viewer a sense of mystery and/or panic: "OH SHIT WHERE DID THAT MONSTER COME FROM IT COULD HAPPEN TO US IN REAL LIFE ANY WHERE ANY TIME". Some of the dialogue adds to this sense of uneasiness, with one character making a point about the fact that humans have barely begun to explore the depths of the ocean.
Yeah, I'm not kidding. I found it a bit hard to be intimidated. Gwoemul's monster was much more awesome. Aesthetically, anyway. I mean becuase, really, if you were to put Gwoemul's monster up against Cloverfield, there wouldn't be a contest.
All my gripes aside, it was a rather enjoyable movie. Just make sure you're not surrounded by teenage couples making out like I was.