Actually I found the HP films to be extremely close to the source material, can't believe I forgot about those!
Mind you, they did have JKR on set to tell them if they were doing anything wrong. I think the HP series are the truest-to-source movies I've ever seen.
If there were any changes, they weren't glaringly obvious as far as I remember, but then I only read the books twice. The films really are rather good, as far as kids films go, and true to the books, they get darker as they go on. I think my only gripe was that the ending of the last film should have been more epic, to really hammer home that "This IS the defining moment of the entire series." I think the big duel between the two main characters could have been strung out, emphasised and gritted-up as for me it didn't really have as much of a kick as I would have liked.
But seriously, if that's my only major issue over a course of eight movies, they did pretty well! :lol: