The film was a gruelling experience to say the least but one that is definitely worth it once you get the third act and the finale.
It reminded me of Seven in terms of the serial killer plot and how that was also quite a grim watch in terms of its unrelenting tone. I Saw The Devil though was much darker with Min-sik Choi playing a horrible character and one which you want to see get his comeupance. His evil deeds are unrelenting, vicious and pretty sick whilst playing the public face of this character is one of a calm nature but with an ever present, underlying social pathic bent. The school scene in particular is one that will unnerve; you know the man and his juxtapostion with the innocent children is unsettling to say the least.
Definitely one I would recommend but I’m not sure if I would watch again just for the sheer endurance involved but you will know that when you do finally get to the finale it will have been worthwhile.