In my opinion, the CBO is one of the best, least-partisan organizations in Washington.
But you do have to understand their limitations. They are number crunchers, not policy analysts. They are directed by Congress, and congress has a lot of power to skew the reports by setting up the assumptions and constraints the CBO must operate under.
For example, the CBO scored the health care bill with the assumption that various cuts to Medicare and other programs would happen, because that’s what the health care bill said would happen. In addition, they were directed to score the health care bill on a ten year timeline, and the bill was structured to front-load taxes to make the ten year score look better. But that doesn’t matter to the CBO - it has to score the actual bill as written, regardless of whether Congress has a history of violating or rewriting bills later. It is not allowed to editorialize or second-guess Congress’s actual performance - and that’s as it should be.
The result is that politicians on both sides of the aisle have managed to use the CBO as a political tool by crafting bills specifically designed to get a good CBO score, knowing full well that the features of the bil that allowed it to score well will be later removed in reconciliation or by amendment.
The bottom line is that you have to be careful to look at the bill itself and make your own determination as to the likelihood of the assumptions in it coming true. In practice, Democrats are good at doing that when Republicans get CBO scores they like, and Republicans are good at doing that for Democrat bills, but when it’s their own bill being scored and they get a score that makes their case, suddenly the CBO becomes the final arbiter whose judgment and numbers are solid fact that cannot be questioned in any way.
Sometimes the CBO will refuse to score a bill if they feel it does not contain enough detail. President Obama submitted a budget for CBO scoring, the CBO threw it back and said it couldn’t be done because it didn’t provide enough detail.
You can also get some insight into what the CBO really thinks by reading editorial comments by the Director, or by reading transcripts of hearings where the Director is questioned about a CBO report. For example, the CBO scored the health care bill as being deficit neutral, but the director later editorialized that the assumption of savings by cutting Medicare and by finding waste, fraud, and abuse was unlikely to occur, so in the real world the Affordable Care Act would likely not be deficit neutral.