I got to see this a while back, and I hate t be an echo, but I agree with you guys - this movie's a mixed bag.
I'll be honest - I went into this one expecting something new and different. Based on "The First", I immediately knew the fight scenes would be good, but not great, the special effects would be so-so, and the story would act like an old car - it starts up nice, but as soon as it gets good, it sputters and slows and takes a while to get back up to speed. "The First", however, did a good job introducing KR1 and 2, and providing a decent backstory for them....even if it was filled with pretty random and out of place story elements(
the two kids, and their eventual shocker turn was defintely a "WTF?!?!" in my book.)
The Next suffers from pretty much the same as The First, and feels like the screenwriters got confused while they were writing it. I agree, this was based on a J-Horror premise, but in the end, it just feels wrong. While I'd LOVE the idea of a Kamen Rider fighting some supernatural being and being so badass that their rider kick can destroy an undead(
Yep, I'm a Kiva and Agito fan 
), the plot didn't help. You mean to tell me a mutated person can go ghost and kill people at random? J-Horror always establishes that the being in question is either dead, cursed, or was summoned.....If she can do it, why can't any other Shocker agent? Why can't V3? He's just like her, right? why doesn't he have supernatural powers?
And Shocker...while I was floored by the fact that the Shocker Riders were in it, they didn't get much good fights in....what I mean is, if they're as good as they make you think, why don't they do more? As a matter of fact, why can't TOEI afford a better stunt team? the fights looked lazy to me, though there's hope for the future, since Kiva has some pretty awesome fight scenes(
some of which feel like they were lifted from the movie and enhanced, mainly the chase scenes and the fight atop the mack truck.)
Fun movie? yes. The best? maybe, if it wasn't for the tacked-on Ju-On storyline.