A good adaptation isn't about whether a film follows its source book to the letter, and I don't think that is the main complaint that people have about the Harry Potter films. I just find that they feel flat, like we're getting a whistle stop tour of some of the main events in the book but there's no depth to it all. A good adaptation can distil the elements of a book to create a great film like the LOTR films or LA Confidential or One Flew Over the Cuckoos Nest (which is very different to the book but that's irrelevant as it's a great film in its own right). I just don't think the HP films do this. There's far worse out there, but it doesn't quite gel for me.