This is an interesting subject. There are some stories that I think couldn't possibly be made into a film. 'Oh Whistle' is one, where I couldn't see how television could capture the nuances of the character of Professor Parker and the sheer terror that he bears witness to. Then I saw Timewatch and Michael Hordern take the story in a different way, and do it quite brilliantly. However there was a more modernised version, and that didn't do too well at all.
I think that the best movie/tv shows are those that don't necessarily stay entirely true to the book that they are based on. After all books and tv/film are entirely different forms of media. The best adaptations are those that use the strengths of their format and try not to copy the parts of the book that would make them weaker. The tv, radio and book versions of Hitch Hikers Guide are all quite different in their own way, but all stay true to the vision of Douglas Adams. The film version is also different, but didn't capture that same magic.
So would 'never say never' about a filmed version of a book (although tv series do tend to do much better than movies). And if a tv series or movie encourages viewers to seek the book to read afterwards, then that is never a bad thing.