There's lots of different opinions on this and lots of valid reasoning for all conclusions. However, I will say that the one thing everybody should agree on is this: The US entry into WWII drastically shortened the war and saved an untold number of British and Commonwealth lives. And for that matter, the bravery and determined fighting of the British and Commonwealth soldiers and civilians saved an untold number of American lives; both in the war and from the eventual conflict that would have resulted had the Germans dominated much of the Western world.

In short, we were and are allies...united in common beliefs and causes. We won the war together, and together managed to keep the Western world largely at peace since then. That's what really matters.

This is just a short answer but, kinda. The supplies being sent across the Atlantic were incredibly benefitial to the British, as was the numerical support given by the American troops.

However, Britain was for the time being relatively safe from an all out land assault after the Battle of Britain solidified the RAF's dominance over the skies.

It's probably safer to say that without one of the three major allied players - the US, the British Empire and the USSR - the outcome of the war would have been up in the air rather than certain and would have lasted alot long. (Without America Britain wouldn't have had the numerical advantage it needed for Operation Overlord. Without Russia all Germany's forces would have been focussed on the western front. And without Britain America would not have been able to reach Europe and all of Germany's forces could have been focussed on Russia).

Basically the three major allied players were all important. Americans often claim to have saved Britain, but the British would respond that they should've joined the war effort from the start...

Hope that helped to some degree

It is difficult to imagine Britain ever retaking the offensive in Western Europe without American financing and equipment, at the very least.

Germany likely would have lost the war against the Soviet Union without a Second Front, but the results would have been far bloodier and taken far longer.