I thought this film was excellent. To understand this film and get the most out of it you need to know about Korean history and people. The story is plausible (certainly by Hollywood's standards) and the action and set pieces belie the low budget. The character of the assassin Hee/Hyun symbolises the divided Korean peninsula and Korean people's attitudes towards each other. While there is division, there is still great love, and regret. This film tries to put that in context. The fact that the event of the film is a football match between the two Koreas, and that the leaders shake hands in the movie, expresses what many Koreans wish for - a peaceful and amicable reunification.
Setting aside the story, the cinematography and tense pace of the film are electric. Korean films are individual, and like their people, culture and food, a somewhat acquired taste. However, those that persevere will be well rewarded.
I would also recommend Memories of Murder, Old Boy, Chihwaseon and Public Enemy as excellent examples of contemporary Korean cinema.