I should say first off that I really enjoyed this film - I didn't really know anything about it, but I know the director from his other work, and I'd heard some good things. It was a thoroughly entertaining watch!
On the good side, the creature, the 'Host', is superb, and it makes a great change to have it out in the open causing mayhem. It's movement and reactions are fantastic and more natural than Hollywood would ever allow, and it looks and feels like a real animal because of it. The first 'rampage' is thrilling, and gets things off to a fantastic start. The main characters, especially the father of the little girl taken by the creature, are brilliantly engaging, and I really cared about the outcome by the end.
There are some big problems with this film. The main one is a distinct lack of really chilling or frightening moments, although there are plenty of jaw-dropping and exciting ones instead! The fact that this film was made in Korea, and by a truly original director, are the main reasons for its appeal for me - the look and feel, the entire approach are different from what we expect, and it keeps your interest. However sometimes the differences distract from the story - the emotions of the characters at key moments sometimes seem very strange and jarred with my own.
Overall I would certainly recommend it - for those who enjoy Asian cinema, for horror fans who don't demand too many 'shocks' or for anyone who enjoys something a bit different from the norm. It's an original, enjoyable but somewhat flawed effort, and I'm not sure if I'll watch it again in the near future.