I am not a fan of Stephen King's writing (though I am a fan of the man), so I have purposefully stayed away from The Dark Tower series.
King's books always seem to follow a simple pattern with me.
The first third of the book I find myself excited, joyfully surfing the book on the wave of King's pure inventiveness (no matter how I feel about his books in the end, it is hard to deny that his crazy mind is full of interesting ideas). In the second third of the book, the wave invariably begins to lose its power, and I find myself growing annoyed. By the final third I am just angry, and the wave is spent while I'm still yards from shore.
The Gunslinger didn't do this to me. I was in that pleasurable first third of King experience for the entire book (I expect, however, that I will continue to feel this way until somewhere in the third book, where the true first third of King's story finally gives way to the second third. The Dark Tower is seven books, after all). The Gunslinger and Roland himself were completely unexpected joys for me.
I loved King's bleak prose (and his prose is rarely something I would praise) because it matched Roland's bleak soul and the books bleak landscape. I loved the fractured narrative that took us to multiple points in Roland's past, while dropping us smack in the middle of his quest for the Man in Black and thus The Dark Tower. I loved Roland's gray ethics, his ability to shoot a woman he'd slept with only hours before, his willingness to sacrifice a boy he loves to fulfill his obsession, his cold, calculating, hardness, and most of all his tenacity.
I am not a fan of good vs. evil stories (and, sadly, I understand The Dark Tower series becomes one). I don't even believe in good and evil (certainly not in the way most people do), so to see a character whose behavior is decisive action motivated by what he perceives as necessity, and action that is (for now) presented outside the values of good and evil, is a refreshing change.
I am sure "theory of thirds" decline will happen as I continue the series, and I doubt that the story will live up to the promise of this, its first chapter, but I think it will be difficult for the rest of the series to taint the beauty of this one book. And I never thought I would say that about any Stephen King story that wasn't a short one.