It's been bad since 5. 5 had a few decent episodes early on in the season, but faltered mostly because of episodes with horrible plots that really had nothing to do with the progression of the show or the mother storyline.
6, 7 and 8 have all suffered from the writers basically going from writing clever dialog and really funny jokes to making the humor juvenile and zany. The characters also no longer feel real and are ultimately unlikeable.. and they've spent too much time on emotional relationship drama.
I stuck it out over the past few seasons because it literally at one point was my favorite show, possibly of all time, until recently. Now I've missed the last two episodes without any real desire to watch them. I seriously hope they end it this season.