I'm kinda close to epaddon on this one, but a little more critical.

I felt the show was excellent for the first 7 seasons (albeit with some stinkers and mediocre episodes here and there). In fact, seasons 6 and 7 were my favorite, as they had some of my favorite individual episodes.

To me, a slow decline began in season 8. Season 9 was worse than 8, but still overall good.

Season 10 was the noticeably worse than 9, and 11 was MUCH worse than 10.

Then there was the abomination known as Season 12, which honestly shouldn't have existed at all.