I only have a sketchy, incomplete impression of what Marx actually said, so far, and this discussion has motivated me to learn more about that. I appreciate the informative comments by those of you who have actually studied the subject. Based on what little I know so far, it appears that probably nothing has done more damage to the understanding, credibility and acceptance of Marxism than the way Communist leaders like Lenin, Stalin and Mao chose to interpret and implement (and probably distort) Marx's ideas. Does anyone more knowledgable than I agree with that?
To be fair, Marx did believe in communism during the first 2/3 of his career. But his theory was based on a Hegelian dialectic of historical materialism which proved problematic, and by the early 1850s, he'd more or less abandoned the notion of communism when it became clear that the proletariate were not moving toward mass rebellion (and indeed capitulated to power as often as not) in favor of union organizing and democratic reform movements (which eventually become European social democracy). My impression is that he would've called himself a socialist at the end of his life, but would've characterized communism as his hope for what could be (i.e., a utopian vision).
What is most striking about Marx from a sociological perspective is his empirical work about the social structures of capitalism, which he called the social relations of production. A lot has changed in the past 150 years or so since his death, so capitalism looks a lot different now (e.g., there's a degree of global interdependence that far exceeds what Marx experienced and predicted), but even the behavior of the financial markets leading up to 2008 follows pretty logically from Marx's descriptions of how capitalist financial markets function and why.
When I teach Marx to undergraduates, I tell them to both read it contextually (i.e., what was capitalism like in 1850s when he was writing Kapital), and comparatively (i.e., what is the same now vs. what is different). In broad general terms, capitalism functions on pretty much the same model, with its own internal logic (profit) driving all social relations and shaping entire societies. I think what would've been fascinating to Marx is that globalization had completely geographically disarticulated the producer class (e.g., China) from the managerial-consumer class (e.g., most of "the west") from the owner class (e.g., the global capitalist class) such that none of them is aware of the other, which functions ideologically on a global scale to keep the social relationships among the classes hidden.