It's good. To my astonishment, against all the odds, it's a good horror film. I'd recommend it. Let's look at how it defied fate:
(a) It's a sequel to The Ring, so it was guaranteed to fail. The Ring breaks sequels. It's so pure and complete as a story that until now everyone who's tried to turn it into a cash cow has just looked as if they're flogging a dead horse. Remakes are fine. There have been lots of those. Ring 0: Birthday was also quite good, but that's the exception (and also a prequel not a sequel).
(b) If you include American, Korean and TV series adaptations, this is the fourteenth Ringu screen version.
(c) Look at that title. "Sadako 3D 2". You might as well announce that you're hiding lepers in the cinema. "3D" and "2".
(d) It's a direct sequel to Sadako 3D, a film so laughably ill-executed that you can't just call it a "bad sequel". It's a franchise-killer. It's Ringu as Scooby Doo. Yes, the Ring franchise needed lots of cartoonish CGI monsters!
This film is by the same director as Sadako 3D and effectively the second half of a two-part adaptation of the same novel. Nonetheless it's good. I liked it. It doesn't go apeshit with dumb CGI, for starters. That's always a plus. Also, more importantly, it's so different from the original Ringu that they've avoided the Curse of Ringu Sequels. The story's nothing like it. It's doing completely different things from its famous predecessor and succeeding pretty well. It's more like The Omen, actually. The previous film's protagonists have had a little girl, Nagi. She's four years old and might possibly belong on a scale somewhere between "unsociable" and "terrifying angel of death". She's sullen and unlovely. She gets bullied by other children. She might conceivably be the cause of mobile phone screens going blooey, in an updated version of the original's cursed VHS tapes. Perhaps as a result, one of this film's main characters refuses to carry a phone. This will be why he avoids being the person who stabs herself in the eye with a knife.
Unfortunately Nagi's parents seem to have gone off somewhere (ahem, cough). Thus she's being looked after by her aunt Fuko (Miyori Takimoto), who's mysteriously still alive even though people all around are dying. This is the film's backbone. The Nagi-Fuko relationship asks some challenging questions. It takes you to a place where you might end up approving of a certain character's decisions, despite thinking that this will probably cause death and apocalypse. I admire a film that can create contradictions like that in an audience.
The genre stuff is well done. There are some startling moments that work. Only one attempted jump shock failed for me, which might be because that was in a dream sequence. This film has the rare gift of doing flamboyantly daft things that should be silly, but instead give the film energy and manage to be scary. If only they'd managed that last time. The film also makes good use of the classic horror device of the Victim Who's Asking For It. You dislike them. You wouldn't mind seeing them die. Ooooh, look, they're being annoying to Nagi, heh heh. (One of them's a classmate of hers, by the way, so avoid this film if you don't like bad things happening to children.)
It's genuinely worth watching, I think, and you don't even need to have seen Sadako 3D. The first half's lots of violent fun, but it's also a reasonably simple Omen-like horror that's giving you less to think about than Ringu and Ringu 0: Birthday. The second half is what turns this into a meaningful film, with Nagi-Fuko character exploration and drama. It's also worth noting that the child actors are impeccable, despite being four years old. That's pretty stunning in itself. They're completely holding their own opposite the adults. Suddenly I'm rather looking forward to 2016's Sadako vs. Kayako, since both Sadako and Kayako have become mothers in their most recent films. (I bet Sadako vs. Kayako does nothing with this, alas, but that could have provided some awesome material to play with.) Admittedly my expectations had been rock-bottom, but I was impressed with this one.