meh, Colbert is funny. But if you are a conservative who is incapable of laughing at yourself, I could see why you wouldn't be a fan. The whole Colbert brand of humor seems to have been a reaction to the Bill O'Reilly, Limbaugh, and other far right wing shock boys.
Never been a fan of Letterman or Franken.
But of course its worth mentioning that we're just talking about our personal opinions here. And I personally do think it is possible to be partisan and be funny, and I do think that it is something that conservatives fail to do for what ever reason. I wouldn't describe them as bitter though...just not funny to me...and apparently to most people.
I suppose you don't care for George Carlin or Louis CK either.