David Leonhardt explains why the stimulus worked in his New York Times piece (h/t Jed):
Imagine if, one year ago, Congress had passed a stimulus bill that really worked.
Let’s say this bill had started spending money within a matter of weeks and had rapidly helped the economy. Let’s also imagine it was large enough to have had a huge impact on jobs — employing something like two million people who would otherwise be unemployed right now.
If that had happened, what would the economy look like today?
Well, it would look almost exactly as it does now. Because those nice descriptions of the stimulus that I just gave aren’t hypothetical. They are descriptions of the actual bill.