Being "older", I got into Harry Poter through my kiRAB. I watched the first two films on DVD and wasn't "into" Harry Potter. I saw POA at the cinema and then read GOF (then quickly bought OOTF after finishing GOF).
Yes there are bits missed out and added scenes but I don't think this makes the films bad because of it.
I've read the entire works of an author called David Baldacci and his first book was made into a film - Absolute Power (directed and starring Clint Eastwood). In this film the main character didn't exist and the character played by Clint Eastwood became the main focus of the film (even though he was killed half way through the book!). I think we should be happy that the films are pretty faithful to the books by comparison!