I do not think the tax cuts are a good idea. Most of the increase in after-tax income is likely to be saved, rather than spent on buying goods and services. One of the reasons why the recession has turned into a depression is that Americans have meager savings, most of them in overpriced houses and overpriced stocks, and so they are sensibly reallocating income from consumption to saving. And there is much evidence that even in normal times, people spend less out of temporary income spurts than they do when they receive what they think will be a permanent increase in in-come. There is no such thing as a permanent tax cut, because the Congress that enacts a tax cut cannot bind subsequent Congresses (there is a new one every two years) not to rescind it.
I also think the transfer payments are a bad idea. The goal of a Keynesian deficit-spending program is to restore demand to X, not to increase it. If instead of demand rising as a consequence of the program from X - Y to X, it rises from X - Y to X + Z, there will be inflation because demand will exceed supply. Programs to transfer wealth are very difficult to abolish, because interest groups form about them. The problem is somewhat less serious with public-works programs, especially road-building and other infrastructure projects, and especially those infrastructure projects that were planned or begun by states or municipalities and interrupted or deferred because of the fall in tax revenues resulting from the depression. The federal government can finance these projects until the depression is over, and then the states can continue them with its own tax money.
There is a legitimate concern that many of the projects undertaken by the federal government will yield costs in excess of benefits. But the concern is exaggerated, because it ignores the benefits that such projects confer on fighting the depression as distinct from simply improving the nation's transportation system or reducing carbon emissions or buying military equipment to replace what has been lost in the Iraqi and Afghan wars. To the extent that the projects by increasing demand reduce unemployment, and reduce fear of unemployment by those who are not laid off (yet), they not only increase people's spendable income (unemployment benefits are lower than the wages they replace) but by reducing job insecurity reduce the fraction of wages that people save rather than spend. The saving rate has soared in recent months and is one of the major factors in reducing consumption and pushing us to the edge of a deflation.
In addition, public-works spending has a multiplier effect. The government's expenditure on buying goods and services (a road, a bridge, or whatever) increases output directly, but it also does so indirectly because the company that builds the project with government funds pays its employees and suppliers, and they in turn spend part of the money they receive, further stimulating output.
Properly structured, a Keynesian program can help to check a downward economic spiral. With monetary policy apparently inadequate to avert a downward spiral big enough to trigger deflation, there may be no good alternative to such a program.
As Posner and others have indicated, there appears to have been a huge conversion of economists toward Keynesian deficit spenders, but the evidence that produced such a "conversion" is not apparent (although maybe most economists were closet Keynesians all along). This is a serious recession, but Romer and Bernstein project a peak unemployment rate without the stimulus of about 9%. The 1981-82 recession had a peak unemployment rate of about 10.5%, but there was no apparent major "conversion" of economists at that time. What is so different about the present recession compared to that one, and to other recessions since then, that would greatly raise the estimated stimulating effects of government spending on various types of goods and services?
It is relevant in answering this question that the origins of this recession were in the financial sector, and especially in the excessive mortgage credit to sub prime and other borrowers. The widespread collapse of the financial sector, and the wholesale retreat from risky assets, clearly has called for a highly pro-active Fed. But it is not obvious why this should lead to greater confidence in the power of government spending stimulus packages. Of course, perhaps the prior emphasis on crowding out, and skepticism toward the stimulating effects of government spending, were wrong, or that recessions were too short and mild after the 1981-82 recession to call for Keynesian-type stimulus packages.
Time will tell whether I am right that a spending and tax package of the type analyzed by Romer and Bernstein may stimulate the economy as measured by GDP and employment, but that the stimulus will be smaller then they estimate, and its value to consumers and taxpayers could be even smaller.