The usual crowd of American high-schoolers + a mysterious notebook allowing its owner to supernaturally kill people undetected + an appending monster providing instructions = Very oddly not at all as silly as it sounds! Actually quite watchable.
Still, a caveat : I have no doubt this all worked a whole lot better in the original manga.
Why do so many filmmakers seem to believe any story is automatically transposable to film? Each medium has its own specifics, it seems to me.