I'm a big fan of King, although his books vary widely in quality. From a Buick 8 was a snore-fest, and there have been others I was equally unimpressed by, such as Duma Key. However, when he's at the top of his game, his ability to write interesting, sympathetic characters may be unparalleled. I adored Jake, Eddie and Oy from the Dark Tower, Barbie from Under the Dome, and the list goes on.
I don't think he can be classified as just a horror writer. There are few, if no, elements of horror in stories such as Rita Hayworth and the Shawshank Redemption, The Green Mile, or even the Dark Tower series itself. A lot of his earlier books are, but his writing started to evolve, and branch off in different directions, sometime around when he wrote It.