In a recent interview with Variety.com, David Heyman was asked several questions on his overall feelings on the ending of the Harry Potter film series, and on its overall success.
CG: Do you attribute [the movies'] success to being faithful to the books and keeping fans happy?
Heyman: (Author J.K. Rowling's) books provide a loyal fanbase, but I also think we made films that stood on their own terms. You never see 'Harry Potter and the Sorcerer's Stone' with film covers. Jo was very keen to keep the books and the films separate. They're very different. Clearly, the success of the books has made the films possible and fueled their success, but it's amazing what percentage of filmgoers did not come to the films from the books.
You can read the rest of the interview here.