On this day, the third anniversary of the signing of the Recovery Act, I can say categorically and without hesitation that I'm pretty sure the stimulus worked, more or less.
Here's the Congressional Budget Office and the Center on Budget and Policy Priorities with graphs showing that the Recovery Act -- aka: the stimulus -- increased real GDP by up to 1.9 percent (first graph) and added as many as 2.4 million jobs (second graph).
Don't believe the CBO or CBPP? There are more studies for you. In Dylan Matthews's monster analysis of nine stimulus surveys, seven concluded that the stimulus probably worked and two concluded that the stimulus probably failed.
Once again, if you're in the mood for certainty, click away now. All of the studies have significant drawbacks. Some studies concluded the stimulus worked with the same models that predicted that the stimulus would work years earlier. In other words, their conclusions had been reached, more or less, before the stimulus was signed. Another study gave the stimulus a failing grade, but it had such wide predictions that "anything between 1.5 million non-government jobs being lost and 1.5 million being created [would be] consistent with the study." In other words, it concluded next to nothing.