I first watched Ju-On: The Grudge (2002) earlier this year and was blown away, I felt rare horror-chills throughout. It became an instant Top 10 Horror Film! A couple of years earlier I saw the american The Grudge 1 & 2: the first was nicely watchable but not scary. The sequel quite meh.
Now I'm on to my third version of the same story, tho' The Curse (2000) was released first. Unlike the american version I got a couple of scare-chills down my shoulders! Just not as often as its bigger-budget remake Ju-On: The Grudge. The lower budget and samey feel of the different sections make The Curse feel less substantial than the epic trippy feels of the 2002 version, the acting is also a level below. It's still superior to the american version.
Fair warning: there is a horribly-disturbing scene involving a baby near the end. It's not graphic in a gore sense (you don't see an actual baby), but it's hard to watch. Neither the 2002 version nor the american-remake had such a scene...it's just a bit much, frankly (i'm a parent of small kids so maybe a bit more sensitive).
Still, The Curse is objectively an impressive low-budget effort, but is only really worth watching if you've already seen the superior 2002 one, and are curious what the makers did before. It's essentially the same story.
I will watch The Curse 2 next, as it appears that Ju-On: The Grudge 2 is its own thing. Tho' not quite clear on that, and I don't wanna stumble on any spoilers by digging too deep.
It can get confusing to keep track, here's the Grudge films I've seen so far:
The Grudge (USA 2004) - 6/10.
The Grudge 2 (USA 2006) - 4/10.
Ju-On: The Grudge (Japan 2002) - 9/10.
Ju-On: The Curse (Japan 2000) - 7/10.
The 2002 one is essential, and should be watched first for maximum impact. If The Curse 2 and/or Ju-On: The Grudge 2 are close to that quality then I may be tempted to check out all of them (there's at least five more related films after these!).