Just How Much Should We Trust the CBO?

In a hyper-polarized political world, the Congressional Budget Office, or CBO, has become the go-to source for politicians whenever they want to argue their cases, because of its reputation of non-partisanship, which instantly confers credibility.
 
Democrats, for example, cite the CBO when asserting that the Affordable Care Act will save billions in health care costs, while Republicans say the CBO agrees that economic growth would be compromised if the Bush tax cut were not extended.
 
But just how accurate are CBO economic reports?