Jonathan Chait offers a defense of the stimulus:
Private forecasters unanimously believe that fiscal stimulus can temporarily boost growth. They give no credence whatsoever to the various right-wing alternative models in which fiscal stimulus does not boost growth. Moreover, in 2001, when the objective case for fiscal stimulus was much weaker, there was no real debate about the efficacy of fiscal stimulus. The fact that Republicans are fiercely contesting the merits of fiscal stimulus now, while almost nobody was doing so when the case was much weaker in 2001, suggests that the right's skepticism is a political phenomenon.
I find this pretty underwhelming, since private forecasters also unanimously think they can make forecasts, a belief which turns out to be not very well supported. More than one analysis of these sorts of forecasts has found them not much better than random chance, and especially prone to miss major structural changes in the economy. Just because toggling a given variable in their model means that you produce a given outcome, does not mean you can assume that these results will be replicated in the real world. The poor history of forecasting definitionally means that these models are missing a lot of information, and poorly understood feedback effects.
Now, you can argue that these guys are some pretty smart economists, and often that's true. But then you have to stack them up against other economists, including some very smart ones who are dubious about the benefits of the stimulus. Their roles as forecasters do not boost their credibility on this topic.