I enjoyed it more than I'd been expecting. It's quite fun. I like both Ringu and Ju-on, so after a while I realised that I really wanted to see the fight promised by the title. The cast are mediocre and the script isn't doing anything intelligent, but I still had a good time with this film.
It's invented a surprisingly convincing reason why Sadako and Kayako should start fighting, you see. If you watch Sadako's cursed video, then two days later she'll kill you. (No, not a week. They've tweaked the rules a bit.) Nothing will stop her. She won't even let you commit suicide, although admittedly some of her killings are capable of looking like suicides. Try to kill yourself and you'll find Sadako appearing in person to, um, express her displeasure. (It'll still end fatally, of course, but at least you won't be cheating Sadako of her prize.)
Kayako, on the other hand, is one of three ghosts in the world's most instantly lethal haunted house. It was pretty bad in the earlier films, but at least they had some patience in those days. They'd have an entire film to stalk their victims... plus of course their victims' friends, relatives and random acquaintances. One film suggested that they'll eventually wipe out Tokyo, or possibly even mankind. This film, though, is mostly Ringu with only occasional Ju-on inserts, so Kayako and Toshio seem to have realised that they don't have much killing time and so they've stopped messing about. You go in the house, you die. You've got a life expectancy of a couple of minutes, tops. There's one exception to that, but in most cases that front door is one-way.
This suggests that you could have fun by watching Sadako's cursed video in Kayako's haunted house. It's not the kind of fun you'd be likely to walk away from, but in the meantime you might get to see J-horror's two most famous ghosts slugging it out over which one got to kill you.
The cast is the film's weak point. They've cast models as the film's heroines, so Mizuki Yamamoto (Yuri) is particularly poor in her opening scenes and Tina Tamashiro (Suzuka) can't even sit up convincingly. Later they'll meet exorcists, all of whom are annoying and deserve to die. (That's deliberate characterisation rather than an acting problem, admittedly.)
Tamashiro is also visibly half-Caucasian, with distractingly huge silent film star eyes, but her parents in this film are both 100% Japanese. Well, there are obvious fan theories to fix that.
I quite like the film's use of its mythologies. Both Ringu and Ju-on have become urban myths, having been around since the 1990s, and you can get university lectures on them. Sadako's cursed video is still a VHS tape. You've got to buy an old second-hand VHS player if you want to watch it. At the end of the day, though, the film's simply rehashing both franchises' most iconic bits. It's even not trying to do anything clever, thus for instance ignoring the fact that these days both Sadako and Kayako are mothers. Similarly the film never brings in the rarely-seen third Saeki ghost, Kayako's abusive husband. I'd have enjoyed seeing Sayako take him down.
The only mythological bits I disliked are trivial, subjective ones. The child actor they've got for Toshio looks wrong to me, while the cursed video is too short.
Personally I think I preferred the Ju-on scenes. They're scarier and the kills are fresher, although Sadako starts surprising us halfway through. At the end of the day, though, I'd been expecting this to be a dull double rehash, so I was pleasantly surprised. Nanami's story ended up impressing me. That was extreme. I found myself looking forward more and more to the big showdown and wasn't even disappointed when it ended up being quite short. There's a brilliantly evil back-up plan after that. So long as you're not expecting too much from this film, I think it's entertaining.