This is how McArdle summarizes his paper:

In the Wall Street Journal, Robert Barro and Charles Redlick discuss the findings of their new paper, which suggests that the multiplier for defense spending is about .6 or .7 at trend unemployment, rising towards one around an unemployment rate of 12%. They think that the multiplier for non-defense spending is smaller–though this is colored by the fact that it’s very difficult to extract the stimulative effects of non-defense spending. As they note in the article, defense spending tends to happen mostly independent of the economy, which makes it relatively easy to see the effect. Non-defense stimulus tends to occur when the economy is already tanking.

Barro and Redlick also suggest that tax cuts are preferable to spending, a finding broadly consistent with the Romer and Romer paper that found substantial multipliers for tax cuts/increases. However, they note that this effect is harder to pin down over long time horizons.

Here is the article in the WSJ, here is McArdle’s commentary on it, and here is Mario Rizzo’s commentary.

Barro: “Defense spending doesn’t help much to get us out of the recession.”
Bartlett: “Barro says government spending doesn’t help much to get us out of the recession.”
HispanicPundit: “Barro says that government spending is less effective than tax cuts.”