Hans,
Among vintage units, there are some that have a poor EER, and others with a better EER. I think anything with an EER of 8 or more is fine, with less than that, it's better to either have cheap electricity rates or for occasional use. There are some older units with a surprisingly good EER. My 1974 Frigidaire pictured above has 9.6 which is almost as good as some new units. And it's built to last!