Keynesian economists claim that government injecting money (where they see fit) into the economy will spur growth and curb unemployment. It may look good on paper, but it rarely, if ever, works. And if it does, it has only a marginal effect on the economy.
Government central economic planning brings up many ethical worries, besides its inefficiency. Primarily that the government will throw money at special interest groups that have supported them in the past, or to certain sectors of the economy for political points (i.e. whether to reward say, union workers, or to persuade certain industries in supporting their brand of government, most often it is the democrats that use Keynesian spending schemes).
The best way to get an economy rolling is to get government out of the way of business, and allow people and businesses alike to keep more of the money they earn. Thus, they are more likely to spend or invest money where they want to. And when this is done on a massive scale millions of times at the "ground level", it is infinitely more efficient than a handful of "experts" looking at things from a distant top-down perspective; trying to guess and gauge where best to invest and spend money.
Personally, I trust millions of self motivated individuals to know where best to spend or invest their limited funds rather than a smug band of elitists with a seemingly unlimited cash flow, where if they make a mistake, they can get scrounge up more money. But if the individual screws up, they may lose their house or something else, therefore they make more cautious and sound decisions.
@Ryan: If Keynesian economics got us out of the Great Depression, why were we still in it, along with double digit unemployment, from 1929 until the early 1940's? Keynesian policies were in effect throughout the entire 1930's under FDR's failed New Deal policies. Almost his entire presidency had double digit unemployment in the high teens, and in some cases in the low twenties. You call that a success? Something that got us out of the depression? Seriously?
It was largely in part due to the war and our selling tons of raw materials, weapons and other supplies to the allied coalition that pulled us out of the depression. Without the war, FDR would have gone down in history as the worst president ever with the worst policies guided by a ineffective system (i.e. Keynesian economics).
Inconvenient truth about FDR's New Deal and the Great Depression:
Why Keynesian economics is a failure: