In case this hasn't been posted yet... Harry Shearer on the show's quality:
"It depends on the script," he says, sounding at first like a man trying his best to be diplomatic. "There are writers' names that, when I see them on a script, I get very happy and look forward to the week, because I know there's gonna be a pretty sound script that is satirical but smart and not just sort of pointlessly parodic, if I may. And there are other writers' names that make my heart sink. It's sort of unavoidable this far along that that's going to be the way it is."
Though he never comes out and says it--It's time to kill The Simpsons--Shearer does say there have been many times when he and the other cast members have had to castigate writers for being too careless with the characters. He mentions one episode in particular: "The Principal and the Pauper," which aired in September 1997. Writer Ken Keeler handed over a script in which Principal Skinner (voiced by Shearer) is revealed to be, in fact, a former "no-good street punk" named Armin Tamzarian--a little plot point that negates seven years' worth of back story. When Shearer saw the story, he was quite unhappy. And unable to do a damned thing about it.
"I said, "That's so wrong. You're taking something that an audience has built eight years or nine years of investment in and just tossed it in the trash can for no good reason, for a story we've done before with other characters. It's so arbitrary and gratuitous, and it's disrespectful to the audience,'" he recalls. "Then it was, "OK, action.'" He laughs. "Really."
http://www.newtimesla.com/issues/2001-04-26/stuff.html
Thanks to Chris Courtois for the link!