My very favorite machine of all time was the Singer Touch & Sew 755. I am sure it had plastic gears but it sewed like a dream. I used it for everything and wish I still had it. It used cams and had the cutest little ducks. Nothing like the machines today but I did really like being able to wind the bobbin without taking it out of the machine.
I know there was a lot of issues with some of the 600 series. I think it was probably because in the later ones they had started using plastic gears and did not have them perfected.