holiday76, your question is too broad I think. There are almost as many answers to your questions as there are C&V cycling enthusiasts. First I'd have to ask, what kind of classic or vintage bikes do you like. For example, you ask whether Schwins from the 50 and 60's were mostly quality: many people on this forum would say yes, they were the best bikes available then while others would rather walk then ride a Schwinn. It would depend on whether you like the heavy, reliable Schwinns vs lightweight racing bikes. Or when you ask, is a bike with Shimano components usually pretty good: Shimano made a wide range of components from complete trash to top of the line racing gear. In the 60's and early 70's they were not well respected regardless of the quality of their components. By the 80's even their lower end stuff was pretty darn good, their mid-range components were pushing the Europeans out of the market, and their top of the line stuff was challenging Campagnolo (ok, some might say kicking Campagnolo's butt but anyway...). So there were Schwinn's that were heavy, clunky but reliable but are "good" bikes in a sense. And there were beautiful Italian bikes with Campagnolo components that were mediocre bikes made with hi-tensile steel tubing and low-end Campy gear. And there were clunkers and jewels made with Shimano components.

If you grew up in the 60's and 70's and have found memories of cycling back then, then Raleigh, Peugeot, Gitane, Motobecane, Atala, Bottecchia, Bianchi, Legnano and many, many others produced wide rages of bikes from basic transportation to professional racing machines.

If you have money to burn, there were some beautiful high-end machines built throughout the years like DeRosa, Colnago, Cinelli, Rene Herse, and again many more.

If you're looking for functionality, reliability, convenience, practicality, etc in an older bike, an 80's bike would be perfect, especially some of the nicer Japanese models from Panasonic, Fuji, Nishiki, and others. By the late 80's some of those bikes were virtually modern in everything save the downtube shifters but I find them generally rather dull in appearance.

As a rule of thumb to which there are occasional exceptions, nothing that came out of the factory at Huffy, Murray, or Ross or was sold by Sears or JC Penny or any of the department stores or discount houses were very good. The were the X-mart bikes of their time. That said, they all occasionally sold bikes manufactured by good European or Japanese companies under thier brand name or even had custom build racing machines wearing their decals but those are the exception not the rule.

So I guess, if you can narrow down your focus a bit, you might get a better answer.