This is a cut above other movies of the genre: genuinely suspenseful, intelligent, brilliantly acted and visually stunning. Yes, the plot can be confusing - but that's partly what makes it pack such a punch. Watch it twice if you can. You'll get almost as much out of watching it when you know the twist than you do from watching it the first time.
Don't be put off by the fact that this film comes from Korea, a country not too familiar to most Western audiences. While there are elements of the film that are culturally specific, the underlying themes are all too universal - guilt, anger, loss, madness and retribution. All of these are handled superbly by Lim Su-jeong as Su-mi, the lead character. Also worthy of particular mention is Yum Jung-ah, who delivers a deeply creepy and unsettling performance as the stepmother.
While it has its scary moments, this is not really a horror flick as most people would imagine it. It's more a psychological suspense story with an element of mystery. It grips you from the start and will keep you guessing until the end - and possibly beyond!