I would rate their performance very high, given they were handed an enormously difficult situation.
I agree that a large stimulus was necessary and they moved very quickly on that. I had some reservations about how the stimulus was done, but I think that most of it was exactly what was needed, especially the part that got money out to people who would spend it quickly. This includes extending unemployment benefits, increasing food stamps, and the aid to the states. Clearly the infrastructure part needed more careful planning and couldn't spend out very quickly.
I don't think we will know ever exactly how much difference the stimulus made because we don't know what would've happened without it. But it certainly mitigated the immediate damage and the precipitous fall of consumption and everything else that was going on.