S&P has been threatening for months to downgrade the US credit rating and it finally did it. Elites of all political stripes united to condemn them for daring to say the emperor has no clothes. The reason you should pay attention to S&P is that they took this action at great political cost to themselves--governments in Europe are talking about stripping their ratings agency monopolies and replacing them with a pliant government ratings committee. S&P bit the hand that feeds it at great political cost to itself. This is a first step towards restoring the ratings agencies' credibility and should be respected.
Instead, the elites are howling about the agencies having no credibility and should just keep quiet.
Much is being made of a supposed error S&P made in referencing CBO deficit projections. Essentially, they used a particular bogus CBO forecast when the CBO would have preferred they used a different inaccurate forecast. This is obviously neither here nor there, but is being used to try to create doubt around S&P's downgrade or imply that they will withdraw it.
Hell hath no fury like a government bureaucrat (or academic) whose umbilical cord is being threatened.