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Zero. It totally destroys everything about a book. I have found though that horrible series can make good TV. Like Twilight or Dexter or Harry Potter. Horribly written trash but popular with those in the center of the bell curve.
In my opinion, the book is almost alway "better" but they're different. A show or movie can be more entertaining for the unit of time you spend on it, but you'll be reading a lot longer. It can also introduce new people to great books. For example, how many people read the Game of Thrones series after the show started?
Movies/shows can be great, especially for well written stories. They have to be different than the books, because they have less time and actually have to re-create what the book only has to describe. They are a good part of the entertainment ecosystem, and even if the book is better. Alternative versions existing do not harm the book. They can exist together.