Wont say much, because I haven't finished it, but of all the scary/horror films I have watched, this is fallen into a unique category.
Before tonight, only twice had a movie/tv show given me nightmares after watching it (that I can remember). Tonight was the 3rd, thanks to Higurashi no Naku Koro ni.
So if you want a really scary story that may end up giving you nightmares, then watch this.