I consider the detailed historical study of the Soviet Union under Stalin and the People's Republic of China under Mao, to be important in the study of communism*. I do not agree with db0, who seems to hold that Stalin and Mao are just as irrelevant to communism as Hitler or Mussolini.
*The study of Lenin is important in understanding practical techniques for achieving a revolution. Lenin died too early, unfortunately, to offer much practical information on how to govern a socialist country in peacetime.
A detailed study of Hitler's Germany is interesting from a general historical perspective, but not very interesting from a practical perspective of implementing communism. Despite the label "National Socialism", we can easily see that Hitler from day one goes on a very different track than modern communists, and all his subsequent actions, successes as well as failures, are deeply dependent on that alternative track.
Stalin and Mao, however, seem like a different case.
The objection that the USSR under Stalin and PRC under Mao never were communist, in the sense of being classless, stateless societies, seems trivial. Since Marx, it's been an uncontroversial understanding that there must be some sort of substantive transition — often labeled "socialism" — between the existing political-economic system (feudalism for the USSR and PRC) and a classless, stateless communist society. And it is unacceptably dogmatic to assert that the USSR and PRC were not true "socialist" societies just because they did not manage the transition as Marx specified.
Both the USSR and the PRC seem to have least started on a track interesting to communists, that they really struggled with many of the issues that any communist must struggle with in creating a socialist government in actual reality. The most obvious and fundamental struggle was with the private ownership of capital, and the use of private ownership to exploit labor. Both the USSR and the PRC created and managed a more-or-less modern economy and dramatically raised the standard of living without the private, competitive ownership of capital. Regardless of whether or not we want to replicate their methods, they decisively proved that capitalism is not necessary. Both the USSR and the PRC also made great progress (although nowhere near complete success) combating the hyper-exploitation of women.
Of course both the USSR and PRC made some enormous, catastrophic errors. No one wants to replicate the famines in both countries, leading to millions of deaths. But these famines can be explained simply by noting that both countries were under tremendous external pressure to industrialize, and their initial poverty magnified even the smallest error into catastrophic results. Once a moderate level of industrialization was achieved, however, the famines stopped. (And it should be noted that famine and pestilence were hardly absent under capitalism in similar primitive circumstances.)
We must assign more blame to the USSR and PRC for their attempts to forcibly inculcate communist ideology. Such attempts were not only unjust and had abhorrent human cost, they were unnecessary and counterproductive. Communism ought to stand or fall on its own, rational merits, not the ability of the government to enforce assent; to force assent makes it impossible to evaluate the rational merits of communism, for good or ill. (Also the capitalist countries have shown that it's much more effective to bribe the intelligentsia rather than threaten them. Intellectuals are not only whores, they're cheap whores.)
To reject that the USSR and PRC were, at least initially, sincere attempts to implement socialism is to say that communists have no more to learn from them (other than that they somehow (how?) started off on the wrong track), positive or negative, than we have to learn from Nazi Germany or Imperial Japan, IBM or Lehman Brothers.
Of course to accept that the USSR and PRC were sincere attempts to implement socialism does not mean that we must approve of everything they did. Nor are we compelled to conclude that their eventual failure (in that both countries became firmly capitalist) means that everything about communism and socialism is necessarily false.