For the purposes of this, I'm talking fiction. Non-fiction/documentaries are in a totally separate category.
All this talk about Hunger Games being made into a movie (and I do mean all this talk, it's permeated everything!) got me thinking about what's lost (and gained?) by turning a beloved book into a movie/series.Game of Thrones, Pillars of the Earth, Dexter, True Blood, Murdoch Mysteries, the Stieg Larsson trilogy, not to mention the many many adaptations of Jane Austen, Charles Dickens, Mark Twain, etc. (For more try this site.)
I like to think I have a pretty good imagination and don't need a movie to show me what I've already pictured for the setting. Plus there are the drawbacks of lost scenes, composite characters, the movie/show going off in a different direction than subsequent books, and bad casting.
But there are positives. There have to be!
The number one pro I can think of is that you get to see everything on the page, all the description and background on the big screen. Or in the case of a series, the expansion of a beloved universe. But what else? If I've read the book and loved it, why would I take the chance on seeing the movie that may or may not be up to the book's standard?
Or is that the point? To see it there, to take that love of a book to another level?