It really depends. I'll use the Anne Rice books as examples. First, Interview with a Vampire was very well done, and although much of the book was left out, it was understandable because of length. Plus, it had her input, so I was confident that it was done as well as it could be at the time. Then they came out with Queen of the Damned. That movie made me so mad!! On the one hand, I enjoyed the movie, but only after I removed it from the series in my mind. If it is thought of as one of the Vampire Chronicles, it is totally wrong. Everything is wrong, even the name. It is called Queen of the Damned, but is a mix up of both that book and The Vampire Lestat. The characters are wrong, as are their back stories. It was just an awful adaptation. So, I would say it just depends on how it is done, and if it is necessary.