His labor theory of value (something he took from classical economist David Ricardo, mind you) is not strictly correct. However it is certainly true that many if not most workers are being exploited for profit, and that on the other hand many owners of capital receive immense income for which they did nothing. Indeed, all the income growth of the top 1 percent since 2000 has been capital income.
Even Marx’s ferocity is an important reminder of the vicious brutality of early capitalism. When today people like Steven Pinker loudly insist that the history of Western liberalism is a calm and steady march towards greater prosperity and decency, Marx reminds us of the soul-crushing poverty, exploitation, and pitched political battles of the early Industrial Revolution. The reason little English girls stopped having their fingers ripped off in power looms, or American women stopped being chained inside fire-prone sweatshops nine hours a day, is not because the working class patiently waited to be reached by the tide of prosperity, but because they joined together and desperately fought the capitalist class for decent pay and working conditions — often facing soldiers and live ammunition in the process.