I didn't read the books.
Prisoner of Azkaban— and to a lesser extent Chamber of Secrets and Goblet of Fire— had a necessary mix of whimsy, humor, color, and charm, and also seriousness/darkness. The series should have stayed that way.
But then first they sucked out the literal color, then the literal light, then the happiness out of the movies. By the time I saw the earliest part of Deathly Hollows, I just stopped caring. Every moment became joyless and hard to watch.
If there's no more discovery of the whimsy of the wizarding world, no more wonderment, no more seeing characters be just plain likeable, why do I care anymore? The series slowly became about friends being jealous and petty, people being double-agents, and about death and loss.
I had to read a summary of the events of Deathly Hollows, because I couldn't subject myself to 4 more hours of misery after the latter two-thirds of Half-Blood Prince. Now I'm just going to enjoy the full LEGO Harry Potter collection game.
The Philosopher's Stone did genuinely feel like we were transported into a new world with exciting new things. But that sense of awe was not replicated later and it slowly worsened until we got to a green tinted Half-Blood Prince.
These days shock and awe is given attention - Dune really did this well, though Foundation also did well with this initially but took it too far and lost substance.
This might be something to look forward to given we have a new Harry Potter series coming, but it doesn't seem guaranteed to produce good results without good direction, story etc.
Also: