maybe the film will be rubbish, and sometimes when you've really enjoyed a book and then are disappointed with the film, it diminishes the book too. however, sometimes the film helps explain things eg. the da vinci code. i found the book quite boring and didn't manage to get half way through it, but really enjoyed the film.
i really enjoyed the tv series of the ladies detective agency too, it completely lived up to the books, it was how i'd pictured it all in my head - the colours, personalities, places.