Did it offer an advantage? Yes. Was it a war-winning, decisive advantage? No.

Individual infantry weapons (i.e. rifles and machine guns) only accounted for a minority of casualties inflicted by the both sides of in WWII. Though that's not to say that small arms weren't important (hence why the Germans developed the StG44 and the G41).

Artillery and mortars were probably the largest single cause of casualties. During the Normandy campaign, roughly 60-70% of British infantry losses were caused by mortar and artillery fire. Machine guns were probably the next-deadliest weapon on the battlefield at the time. Infantry small arms came a distant third.

First off, I think this is the kind of thing that will be always be debated. In a war where the bolt action rifle and submachine gun were the two primary infantry small arms (as opposed to crew served weapons, or light machine guns), the M1 certainly stood out. I am inclined to believe it gave the US a fighting edge.

If you ever get a chance, shoot a bolt action rifle and an M1 at the same session. You'll likely be as accurate with either one, you'll find that if you do your job, and the rifle is in good order, you'll be able to place rounds in decent groups. You'll also find that the M1 gives you three more rounds than the traditional 5 of the bolt rifle (yes, I know SMLE, and it's faster rate of fire, it's the exception, rather than the rule).

Now, consider not only your higher rate of fire, but those three extra rounds. You have an edge if you know how to use it. You are acquiring targets or reacquiring targets faster than your enemy, you are shooting at those targets faster, and with less time to stay sighted on them. This is an advantage no matter how you slice it. A US Army training video claims that the M1 can put out the firepower of nearly three men using the "old time" Springfield bolt action rifle.

Given the higher rate of firepower, I am inclined to cast my opinion in favor of the rifle giving the US soldier an edge, as the old axiom of "nobody wishes for less gun in a fight" tends to hold true. Many nations were experimenting with semi automatic rifles before WWII, but the US was the first to go into production to rearm their military. The USSR was next, with the SVT 39 and 40, but mechanical issues, production issues, and the invasion by Germany shelved that project. I'm inclined to believe that if WWII had not started for several more years, we would have seen at least the USSR moving to equip their forces with the SVT, and greater examination of semi automatic weapons by other nations.

The reason infantry weapons moved away from bolt-action to semi-/automatic is because it's superior for individual infantry weapons.

I believe there was an after-war study which showed that if factors like terrain advantage, tactical competency, and fitness were held equal, then a lesser number of American soldiers could establish fire superiority over and completely dominate a German team armed with bolt-actions.

The Garand was superior to the bolt-action rifles being used by other armies in terms of rate of fire and magazine capacity (British LE carried 2 more rounds, though). Accuracy and weight were comparable. It was only worse in terms of reliability, at least as compared to Springfields, if it took in water and sand during beach landings, but that was mitigated by wrapping the guns for such missions, and their firepower advantage outweighed the reliability concern. It had one idiosyncracy: the en-bloc clip would eject with a loud metallic noise when ammunition was expended, indicating to enemies that a reload was necessary. I've never heard of anyone having their position rushed and killed because of it. I'm sure it must have happened once or twice, though. But American soldiers actually began using that "feature" purposefully as an effective trap from early on, so even that issue isn't really something that I hold to be criticism but more something to be ambivalent about.

For the reasons given above, I'd rather be carrying a Garand than any other service rifle, except possibly the American carbine, if I were a footsoldier. But that doesn't mean it was a war-changer. German formations had overall greater firepower than comparably sized American formations due to having more organic artillery, which really is more important. Battles aren't won by mere tactics if the enemy has a decisive operational edge, and no amount of battles won will win a war if the grand strategy is unfocused or counter-productive. The Garand at best provided a mere tactical edge. Important, notable, but not war-changing.

The M1 also made a distinct "ping" as its cartridge was was ejected. Offering a disadvantage since the enemy would know you were reloading.