It is the first part of the Vengeance trilogy, followed by Oldboy which is an award winning work and then Sympathy for Lady vengeance. All three movies are on theme of revenge, violence and salvation. It was taught in my course of 'Appreciating far eastern cinema'. A great movie not only from an art perspective but also for those who would like to watch for entertainment.
But yes there is a caution: It contains some violent and disturbing scenes so if you are too sensitive on that, I would recommend you to avoid watching the movie.