For me, it's powerful because it's written first person, which I always find captivating, and its a good old fashioned ghost story. It is one of my favourites, and it scares me to death, but that's powerful writing.
One thing about King, even though this book, like many of his others, is a stand alone story, it doesn't escape being linked with the rest of his works somehow (in this case, Jo had been reading a Bill Denborough novel at one point) and I love that about him, the way he links up all of his books and characters recur making his whole universe seem real.