Banned in China: The tracts are, in fact, banned in China. Also, a couple was found guilty of deliberately mailing certain anti-Islamic tracts to Muslims in Singapore, and convicted of sedition; the bookstore which had imported the Chick tracts for distribution was also dealt with. Some of his tracts have also been banned in Canada as hate literature.
Specifically, he wasn't directly railing against religion as the quote may suggest. At the time, opiates were legitimate painkillers and not illegal narcotics. He saw religion as false, but necessary and useful for the time being. He argued not that a lack of religion would make the world perfect, but that a perfect world, once achieved, would no longer need religion. For obvious reasons, many dispute both his idea of a perfect world and whether any such utopia could ever be achieved. To a modern reader, it seems as though the quote is accusing the ruling classes of deliberately engineering religion as an "addiction" to control the working class. That was not Marx's intent.