Seasons 9-large parts of 13/14 are pretty darn bad with a few really bright spots left in. Homer VS The City of New York is HILARIOUS. I'm hoping the World Trade Center gag is left in on the DVD's. I remember falling all over myself laughing at that. However - no matter how many times I see it, "The Principal And The Pauper is still bar none the worst episode ever created for that show. The creators even know its bad referencing it a few times here and there in jest.
I've lately gotten back into watching it again and have discovered that seasons 15-17 have actually had some really good episodes. There are still a few mediocre ones in there but not anywhere near as all around bad as prior ones though. The show seems to go through phases of absolute brilliance VS periods of "WTF were they thinking allowing this one through"
Jam it back in, in the dark.