Colbert’s vicious use of irony served to show what a chummy, buddy-buddy, insular, self-congratulatory un-virtuous cycle the press' relationship to the White House has become. He clowned the President, clowned the press, and basically dared say what about two-thirds of the country has come to realize.
Incidentally, it’s a really rough thing to watch. The jaw-dropping from the correspondents and the “is he really saying this?” look from the roastee is priceless; painful, but priceless. I think this may be the real tipping point.
Check out the footage or transcripts here.