From Mario Rizzo at ThinkMarkets:
In recent months – or has it been years? – Paul Krugman and Brad DeLong have been saying, in effect, “We told you so – the stimulus was not enough. Look at the sluggish economy and high unemployment rate.”
They are arguing that the problem with the fiscal stimulus is that it was not enough. The idea was right but the quantity was wrong.
The basic problem with the quantitative claim is that it skirts some real problems in the analysis.
The rest is here and definitely worth reading.