Marx's Das Kapital is utter bullcrap, and makes Joseph Smith's Book of Mormon seem like Euclid's Elements. It was before the marginalist revolution, so did not understand 'value', instead thinking all value came from labor. To note how stupid this is, think about someone spending 10 hours writing a paper--does this make it worth '10 hours' worth of 'steel'? Depends on what came out, and probably its worth is inversely correlated with the time spent on it. You can see a nice summary of its irrelevancy by going to the Wikipedia on volumes 1, 2 and 3, and note that the economic arguments are not even used by Marxist economists today (falling rate of profit? recessions becoming wider every time? Surplus value?).
They say sure, he got the details wrong, but he had great vision. Most of the book was very specific, and those little examples and arguments were all wrong. I guess by some cosmic analytic karma, if all your particulars are wrong the theme must be really profound. Literary types have always like it because it gives a scientific pretense to their statist and egalitarian instincts (based on hope alone, as all socialist enterprises turn out to labeled 'state capitalism' or some other oxymoron). Yet I like it when people like Mailer make such points, because it lays clear how hollow their base assumptions are, because it is simply impossible to 'think more clearly' after reading such a book.