I'm not sure the quality of the tank crews has to do with MajorBloodnok's actual question (how good was the Sherman?)
The answer to that? It was good enough.
The 75mm gun couldn't penetrate Panthers or Tigers without getting dangerously close to them but Panthers and Tigers only made up about 30% of the German tank forces as of D-Day. The rest was StuGs, Panzer IVs and various older designs. The 75mm gun could handle those just fine.
A good tank is an adequate tank you can build a lot of. The US could build a lot of Shermans and they were good enough against anything that wasn't a Panther or Tiger.
The Firefly carried a 76.22mm gun (which was the equivalent of the UK 17lber)
The Firefly's gun
was a British 17 pounder. It was a British tank.
The British wanted to mount the 17 pounder on a tank and
their attempts to mount it on a Cromwell weren't going well. So they used the Sherman.
It was a very awkward vehicle. Outside of a battle they had to point the turret backwards so the gun barrel didn't stick in the mud when the tank drove over a bump. The gun was so big that they had to weld a box to the back to the turret to fit the radio equipment in. The main gun's muzzle flash (for which the tank was nicknamed) was so bright that the crew had to close their eyes when they fired or be temporarily blinded.
But it could reliably knock out anything the Germans had and for that it was invaluable.