IMO (which is all this is!), Season 9 was only a half-step better than 8. They did rectify some of the worst blunders of that season (getting rid of the Robert California character, for instance) though there was more that they should have done (the Nellie character never should have been kept around--unlike some of the other posters here, I never warmed up to her). But for every good thing Season 9 did (showing the documentary crew, the addition of Clark Duke as Dwight's protege, and, yes, the last handful episodes were all good choices) there was a terrible, head-scratching decision made (changing Andy's character into an out-of-control tyrant, breaking up Andy/Erin and pairing her with a background character and then completely dropping that plotline, and the introduction of Brian into Pam's life were all horrible story choices). So it's a little better than Season 8, but still well below all the other seasons.
For me, the show just didn't work after Steve Carell left. I suspected at the time it wouldn't, but was definitely willing to give it a chance. He was the heart and soul of the show, and, though I didn't realize it at the time, he kind of gave it a bit of narrative direction, and when he left, the show went from my all-time favorite to...something far inferior. I have never really grown to like the last two seasons. Again, just my opinion.
Dwight was fine in the first three seasons, but really beginning in Season 5, they took him way too far. That really hurt the show, taking a character who most of us know a version of and turning him into someone that nobody can possible relate to.
Yep--they could have redeemed this story by keeping Andy with Erin, but no, they had to destroy that relationship too.