What impact did America joining the war have on Allied troops?

The entry of America into World War I in April, 1917 had three main effects on the allied cause, of which the actual appearance of American troops on the battlefield was the last and perhaps least important, although still significant. First, America's declaration of war on Germany unleashed its navy against the German submarine fleet. American destroyers were free to escort convoys, hunt the submarines, and lay minefields that soon strangled the German effort. That allowed a safer and larger flow of much-needed cargo to Britain and France, carried on merchant ships American shipyards were building at a prodigious rate.
Second, America was able to provide financial resources to the Allies, who had spent enormous amounts of money in carrying on the war. In fact, part of the motivation for the US government to support the Allies was to ensure that their huge debts to the US could be paid later, which they might not be if Germany won the war.
Third, the vast manpower and materiel resources of America reached the battlefields in France at just the right time in early 1918, when Germany was making its last desperate attempt at a breakthrough. Although German forces were badly depleted and overextended, the Allies were equally exhausted, and needed American help to stop the German offensive.
Source: Basel H. Lidell Hart (1930), A History of the First World War. London: Cassell.

In 1917, the Allied powers were very happy that the Americans had finally joined the war. Prior to the Americans joining, the French army had been pushed to the breaking point, and the French soldiers were starting to mutiny in the trenches. While the situation was partially helped with more troop furloughs home and less time in the front trenches, the situation still looked bleak for France.
Britain was happy as well because the British military leadership knew that the war could not go on indefinitely, given the kind of casualties the Allied armies were withstanding in each campaign.
The initial hope in Britain and France was that the Americans would serve in the trenches as fill-ins for British and French troops. When the American Expeditionary Force commander Jack Pershing insisted on his troops serving as an independent army, the rest of the Allied leadership was taken aback because it appeared as though the United States would not be a "team player."
When the Americans joined the fight and served admirably in the Meuse-Argonne Offensive in 1918, the British and French troops admired their bravery but were shocked by the heavy casualties the Americans took in a few months of fighting. While American munitions were more prevalent in the four years of World War I, it was the American serviceman who provided the valuable boost to morale at the end of the war that allowed the Allies to win.