6 is underrated. It's definitely worse than 1-4 by a wide margin, but it's still a good book. Even 5 is passable. Series doesn't really go into a full nose dive until 7, and 8's just a clusterfuck of bad writing and a bad story to go with it. I will never not be mad about the pure literary waste that was Opal's clone.
Having said all that, the movie is worse than the books. As bad as book 8 is, it's still only a 4/10. The movie's a 2/10 at best. ARTEMIS'S MOTHER IS DEAD AT THE START.