OK, so I watched this at 1am with all the lights off and my headphones on and all alone in my apartment. And I have to say, I damn near soiled myself towards the end. On many occasions I found myself holding on to the edge of my sofa. Its that scary. And believe me, I don't have that reaction while watching a horror movie very often (extremely rarely in fact).
A word of caution though. This one really requires patience. You need to immerse yourself into its world. I watched it another night with my girlfriend and she got bored and gave up about half-way. I can imagine many folks doing the same. This is that kind of a movie, it will either scare you silly or bore you to death. I fall in the former camp.
I won't spoil the story for you (as if you didn't already know bout it from browsing the IMDb boards) but there are a lot of seemingly random events happening on screen which make a lot of sense once the movie reaches its horrific conclusion(s). That last scene still gives me shudders.
So watch this with an open mind and give it a fair chance. Paranormal activity, Rec., BWP, and all the other shaky cam brethren have NOTHING on this one. Noroi has them all licked.