Marx's followers, like his critics, are often more motivated by ideology than anything else. You have to read Marx skeptically because 1) He was writing in a time when conditions were very different, and 2) His thought is often guided by historical and sociological assumptions which are quite questionable. On the other hand, he did make many contributions to an understanding of how economies work which are invaluable.

For example, his critics often attack him for missing all the insights provided by Adam Smith, but the truth is, Marx had great respect for Smith and often referred to him. He did not refute Smith, on the contrary, he cited him frequently.