This is the Japanese movie that stimulated Sam Raimi to do "The Grudge" with Buffy. I didn't go see The Grudge based on several recommendations to see Ju-On.
I have the DVD. And I must now inflict it upon my friends at some point. It's really good. Really really good.
From a filmmaker's perspective, Japanese films have a much different style than American films. The acting is different: the actors have almost slapstick performances in a couple of moments, like live-action anime. The look is different: Japan tends toward much brighter colors and more brightly lit sets. That much of this film takes place during the daytime and it's still scary is a tribute to the filmmakers. The pacing is different. It adds a subtle alien quality that only serves the story in my feeling.
It's also a great film to learn from. It looks to be a really low-budget movie. There are no amazing special effects, no CGI monsters, just straightforward storytelling.
I don't recognize any of the actors. Regardless of whether they are big in Japan, they are to me unknowns and I am not distracted by the "star power". I really like that.
There is a commentary track with Sam Raimi where he evidently talks about the things that inspired him while watching the movie. I haven't listened to the whole thing, but what I have listened to sounds well worth it.
It's cheesy. It's got George Clooney in his bad hair days, at least for a short time. It's very 80's. It has Marcia Brady. Did I mention that it's cheesy? Smell the cheese. Mmmmmm, cheeeeese....