Did you see this or the remake first?
I've personally never seen "The Uninvited" (though I've heard it's decent) so obviously the original for me. I think the order of which you see them really impacts your view.
I was breath-taken with the depth and drama of this movie, but after two friends and I watched it, all they did was compare it to the "The Uninvited" which I find unfair. Because they knew of the twist at the end they weren't emotionally involved throughout. And even then when I explained to them the step-mom thing (in which Su-mi is creating scenarios in her head where she plays herself and her step-mother to be there for Su-yeong) they didn't seem to care. It's my understanding "The Uninvited" didn't delve this far in, but still they were dispondant.
They always say whichever you see first you like more, which is really too bad for anyone who saw the remake before this gem.