How good were the relations between the Allies at the end of World War II?
The answer to this depends on which sets of allies you are asking about. Some of the Allied Powers had good relations with one another while others did not.
In general, the relationship between the United States and the United Kingdom were very close and very good. The two countries had cooperated closely during the war and were on very good terms. These two countries and France were also on decent terms. The French, under de Gaulle, did have some agreements with the English-speaking countries. The French wanted to treat Germany more harshly. They also were very concerned that they should get full credit as allies even though they had not participated as fully in the war.
These three got along well enough, though, that they ran their occupation zones of Germany jointly and generally had only minor disagreements. However, relations between the English-speaking countries and the Soviet Union were quite bad. Essentially as soon as WWII ended, the Cold War began. The Cold War stemmed from the serious distrust and ideological differences between the Western Allies and the Soviets.
The relations between the Allies at the end of World War II could be said to have been particularly strained. This was especially so between the Western powers, in particular the Americans, and the Soviet Union. The basis of such problems arose from the fact that all four nations had only agreed to work together due to their mutual fear of the Nazis. To worsen matters, the prejudices that had developed before the war returned, marring the ability of the nations to trust each other and clouding their understanding of one another's policies. Differences also arose over the reconstruction of the post-war international order - the goals of the different nations were clearly incompatible with each other and each side sought to push for their own agendas. The only thing that held them together was their mutual desire to defeat Nazi Germany and once that threat was removed after the war, the alliance fell apart. This ushered in the start of the Cold War period between the Americans and the Soviets.