He contributed a great challenge to economics with his ideas, having to work towards rebuttal and debunking of his simple, yet appealing models.
It seems to be standing academic scrutiny today, at least, it is debated to be. Kliman's[0] Temporal Single System Interpretation of Marx, upheld by Alan Freeman, David Laibman and Guglielmo Carchedi is one example which also attempts to solve for the transformation problem. On the other hand, Paul Cockshott has written on the empirical validity from the opposite pole[4] (note, one which Kliman rejects). Anwar Shaikh takes up what (I believe to be, my memory is rusty) a neo-Ricardian approach to the empirical strength of Marx's theory of value[8]. With regard to Marx's falling rate of profit, Okishio's Fundamental Marxian Theorem is currently upheld by Dong-Min Rieu (in several interpretations)[10]. Recent work has also been done on the empirical validity of the TRPF using actual econometric data[9]. Marx's theory of exploitation is currently upheld by John Roemer, Roberto Veneziani and Naoki Yoshihara[1]. Marx's "third thing" proof is currently upheld by James Furner[2]. The interpretation of the labour theory of value in the causal-explanatory mode is discussed since Steven Fleetwood[3]. The Sraffian criticism of Marx is also very much discussed[5][6][7]. There is also, of course, the Neue Marx-Lektur school which takes the sociological and cultural approach to the theory of value, the relatively new idea of the "value theory of labour", and the philosophical reading of capital such that price and value are different levels of abstraction and thus irreconcilable.
My point is that to say Marx was "wrong, really wrong" does not do justice to the fact that he was wrong on some counts and right on other counts, and your dismissal is much too heavy handed to dismiss much of the work and debate done in political economy.
[0] "Two Concepts of Value, Two Rates of Profit, Two Laws of Motion," Research in Political Economy 18, 243–67
[1] http://www.ier.hit-u.ac.jp/~yosihara/Reexamination_of_the_Ma...
[2] Furner, James (2004) 'Marx's critique of Samuel Bailey', Historical Materialism 12(2)
[3] Fleetwood, S. (2001) 'What kind of theory is Marx's labour theory of value? A critical realist inquiry', Capital & Class 73
[4] http://users.wfu.edu/cottrell/eea97.pdf
[5] Freeman, Alan, (2002), Marx After Marx After Sraffa, MPRA Paper, University Library of Munich, Germany.
[6] http://ricardo.ecn.wfu.edu/~cottrell/ecn265/value.pdf
[7] Bowles, S., and Gintis, H. (1981): “Structure and ractice in the Labor Theory of Value,” Review of Radical Political Economics 12,pp.1-26.
[8] "The Empirical Strength of the Labor Theory of Value" in Conference Proceedings of Marxian Economics: A Centenary Appraisal, Riccardo Bellofiore (ed.), Macmillan, London.
[9] Basu, Deepankar and Manolakos, Panayiotis T., "Is There a Tendency for the Rate of Profit to Fall? Econometric Evidence for the U.S. Economy, 1948-2007" (2010). Economics Department Working Paper Series. 99.
[10] Rieu, D.-M. (2009). Interpretations of Marxian Value Theory in Terms of the Fundamental Marxian Theorem. Review of Radical Political Economics, 41(2), 216–226.