Well, the joke goes that Marx had good and original ideas.
But what’s good isn’t original, and what’s original isn’t good.
To be honest, though, I haven’t read Marx, except the most famous parts of the Communist Manifesto.
I would say that — just like Adam Smith, for that matter — he’s mainly important as a historical figure and a stopping point on the path of the history of ideas, rather than as a direct source of insights.
Now, have any of those ideas been developed into something still relevant today? Well, I’m not sure. I’ve heard that he had a foundational influence on sociology. But I’ve also heard that sociology is a bunch of arbitrary left-wing assertions that aren’t founded in evidence. 😉
More Marx than that.
Marx, as a historical figure, is pretty important. Not for what he did, but for what he inspired others to do.
Marx, as an economist, was not without merit – for example, the idea that different arrangements of society are ideal for different groups of people was not widespread before Marx.
But if somebody is using Marx or his works as a source of authority for their policy choices today, they’re religious, and their religion is Marxism or one of the heretical splinter sects. They have nothing of value to bring to the table, and should be disregarded entirely except for such concerns as might arise under Realpolitik.