I am a confirmed admirer of the films, but I do agree that they do not hold the same "magic" as the books. I think that this is an inevitable consequence of the transformation of the story to the silver screen, not just because of the alterations to the story and characters, but also (primarily?) because (as others have suggested) the "enforced" visualisation (marvellous though it was, in my opinion) robs us of our own opportunity to visualise the events and characters portrayed. Like others, I find it hard to put my finger on it any more than that, but it is perhaps related to the concept of "enchantment" (or loss of it in the film-making process) discussed
here.
I do not, however, believe that seeing the films robs us of our ability to be enchanted by the books. At least it has not for me. Perhaps it might be different for someone who saw the films before reading the books. Anyone care to comment?