How compatible are Buddhism and Communism? Despite communism as practiced (as opposed to what Marx might have had in mind) being the source of some of the largest slaughter of lay and monastic Buddhists ever (except maybe the Imperial Chinese)... are the principles of Buddhism and communism (or socialism) compatible?

Did the historical Buddha or the later sutra writers hit on any of the same concepts that Marx did in any substantive way?
Marxism is strictly materialistic. It builds on the idea that if you distribute all goods evenly, people will stop suffering. That was a reasonable assumption to make when the vast majority of the population suffered from uneven distribution of goods.

Buddhism on the other hand does not regard changing the outer conditions as being important in overcoming suffering. More precisely, it suggests that transcending the outer conditions is the key to overcoming suffering, so in essence it denies the core question that Marxism seeks to give an answer to. 

So I would say that they are pretty much orthogonal. Buddhism could probably act as a formidable complement to the Marxist theories when trying to build a society. But to individual Buddhists, Marxism is not necessarily useful.