The evidence to support this argument is fairly strong. The socialist governments of various northern European governments in particular managed to create societies in which personal liberty, income equality, low unemployment, strong public welfare systems and a high standard of living all coexisted for a long time.
The problem is that this evidence makes a better case for regulating capitalism than it does for socialism classically understood. The sources of wealth in these countries are still preponderantly capitalist. As international market pressures have drawn these countries into a more competitive mode, almost all of them have been dismantling large parts of their welfare systems and selling off nationalized industries to stimulate their economies.