You have to consider how different Japanese culture is from American culture. Horror, like anything else, is culturally relative. Ju-on was hugely successful in Japan, so it apparently scared the shit out of the Japanese. Maybe they'll think Saw blows.
My friend watched Saw last night at a sneek preview. She said it reminded her of Seven.
The Grudge sucks pretty bad. It felt like the writers were just tired at the end and just said "Oh hell, lets just end it here, why not". Lame rental at best.