It's pretty good. It's not extreme, mind you. It's nowhere near the goriest horror film you'll see, while it's not particularly nasty either. What it does have, though, is a baddie who's targeting children... and I'm not just talking about the "Hollywood teens" you'll see in something like Nightmare on Elm Street or Friday the 13th. The film uses actual children.
Then, on top of that, it's tackling themes of child abuse and domestic violence. This gives the film weight, despite its relatively vanilla protagonists.
Unlike its 2005 pink film predecessor, this is a proper kuchisake-onna movie. She wears a hygiene mask. She'll ask you if you think she's pretty. (Hint: under the mask is something that really isn't.) She also has a huge pair of scissors that you could use in sword fights. What's more, the film presents something a lot like the panic of 1979, when young children sometimes weren't allowed to walk home from school unaccompanied. If you had child abductions happening in your school and sightings of a woman dressed as a murderous ghost from folklore, you'd be wanting to keep an eye on your children too.
Then we get into the child abuse. (None of it's sexual, incidentally, instead being about mothers who hit their children.) Classmates are as sympathetic as a nest of vipers. The teacher tries to help, but her handling of the situation slides from "well-intentioned" to "this person has issues". The most challenging part of this is the film's characterisation of the mothers themselves, which I can imagine some viewers rejecting as going too soft on abusers. Mummy hits them, but she loves them too! If her child disappears, she'll be distraught and spare no effort to try to find her little darling! Personally, I thought this was the best part of the film. It's portraying feelings that are complex and often self-contradictory. Eriko Sato plays the film's main character and heroine, but she's also a Japanese mother who didn't get custody of her daughter after a divorce. That doesn't happen often. The film never digs too deep into why, but her daughter appears to be scared of her. Chiharu Kawai's character, similarly, is so busy blaming herself (rightly) that she's got no blame to spare for the questionable actions of the teacher. Miki Mizuno in that flashback will alternate between potentially lethal violence and ordering her son to do even worse to her, because she loves him. "Because the next time I go crazy..."
The kuchisake-onna herself has various forms. Sometimes she looks like a burned-up undead monstrosity that could never pass for human. At other times, you wouldn't have guessed until the mask came off. Her mouth varies too, being sometimes a comparatively subtle practical effect and sometimes an impossible CGI nightmare. She also has a ghost gimmick that gives the film one of its most horrifying moments... and I don't mean that in a horror movie way. It's worse than that.
She's a pretty impressive horror movie monster. She's an odd combination of vulnerable and literally unkillable, with powers that mean (for instance) you can predict the movie's final black twist a good half-hour early. In a good way. Plus, of course, she's a serial killer of children. This film in some ways conforms quite closely to the serial killer thriller genre. (The kidnap victim, the basement, etc.)
The film has its flaws. It's not particularly interested in its protagonists, while there's a CGI gore shot so terrible that you might end up deciding it's the visual equivalent of an unreliable narrator. Worse still, it's in one of the film's most important scenes.
This isn't a loud, brash horror flick. It's a relatively quiet J-horror, but it's being understated about an intensely emotional topic. I'm sure some viewers will find it dull, while others will find it getting to them even in little scenes where nothing's happening. It's quite clever. (I liked its play on "kirei" vs. "kire" in Japanese, although there's no way of conveying that in English subtitles.) It had the option of taking its themes further and I think you could argue that it stops halfway, but it's a horror film that rewards thematic analysis and discussion.