I'm sad to say I was disappointed with the American version of the Grudge series in terms of scaryness. But nonetheless, I liked it because it creeped me out. I've yet to see the Japanese versions (Ju-On), but I dare not because Asian movies have a reputation to be far more scarier than Western Horror Movies in more ways than one.
I've seen the original Japanese Ring movie and after watching it the first time with lights on in broad daylight and with some friends, I could not sleep well for over a week because the character of that movie, Sadako, haunted me for a very long time.
I hear a third Ju-On movie is in the works, but production for Grudge 3 is halted due to problems as to how the story can be progressed with the protagonists from both movies already killed off.