Among Stephen King's large body of work, there are admittedly a few duds, but in far greater number are the really super pieces of work, the great books that will stand the test of time and, eventually, he will be deservedly remembered for. There are at least 5 which could vie for the "his best book" accolade, and Gerald's Game is one of them. It is, in a word, superb.
It is one of King's most eccentric books, in terms of the rest of his work. There is no horror here at all, at least not of the ghouly kind. Instead, we sink down into the mind of one character, trapped in a cabin for four hundred pages, and see the internal horrors that lie within it. It is a psychological novel more than anything, which would most probably appeal to fans of authors like Ruth Rendell or Ian McEwan - fans of writers whose normal subject is the human mind, its messinesses and ways of preservation, its internal conflicts and idiosyncrasies. Gerald's Game is a excellent book because King is able to keep us in the company of one single character, alone in a room, tied to a bed, for almost the entire book, and never do we experience a moment of boredom. It would be difficult to overstate the level of achievement this illustrates.
Obviously, not everyone's going to like it. There are people who will find this book boring, and will want more action, more obvious thrills and excitements. If you like subtlety, though, more focused and mental thrills which spring from a more psychological well, this book is for you. It's a marvellous exercise in form and structure, and an absolutely gripping psychological novel. I think King's best work is normally produced when he moves away from overt horror, and this is an example of that. It's a sharp, deeply engaging and gripping book that is no less terrifying than if there were vampires or ghosts involved. Read it. It's not only unique among King's work, but in almost everything else I've read as well.