[quote name='tranceFusion']I stopped at 9.. are there still more that are worth it after that?[/QUOTE]
Many people, myself included, think the episodes really start to suffer in quality around the Al Jean era, which was around season 13. He became the Executive Producer there.
I really think they lost it after 13. Part of me still wants to support them from 14 on, but part of me says enough is enough. I have every box set except 12, 13, 20, and given that 20 is rated very terribly in both quality of the set and quality of the episodes, I'll likely pass on that.