Observed Performance and Effects of Communism
One of the primary observed effects of a centrally planned economy is a general weakening of economic performance. Measured by output and income, centrally planned economies tend to be less efficient free economies where output targets, and the means to meet them are made by a private sector. However in some sectors, Communist nations such as the former USSR have led the West in the strength and quality of some defence and space technologies at times in history.
Famine and Genocide
Many communist nations have suffered periods of famine and genocide, although sometimes the deliniation between famine and genocide is not clear.
The Ukraine, for example, lost approximately 2.5 - 3.5 million people to famine in 1932 and 1933, the so-called Holodomor. Most modern historians believe that this famine was caused by the radical economic changes, including collectivization of agriculture, of Stalin's policies, and the seizure of crops by the Soviet authorities.
The Great Chinese Famine of 1958-61 (also known as "The Great Leap Forward Famine") is estimated to have cost the lives of between 14 and 40 million people. Chinese authorities had intended to increase national food production while also increasing industrial capacity, but the result was famine instead. The Chinese communists attempted to reorganize farms into "collectives", which suffered from great inefficiency and thus an overall reduction in food production. The full scope of the famine, as well as the details of its causes, are still not fully known. 
In Cambodia, when the Marxist Khmer Rouge (literally "Red Khmer") assumed power, they began to empty the cities of people on the pretext that the cities were about to come under American attack. The communist leadership knew this was not true, however, and the operation was intended to depopulate the cities, as they were believed to be living, breathing centers of Capitalism. The Khmer Rouge referred to Phnom Penh as "the great prostitute of the Mekong." The people were shipped to farms in the countryside ("The Killing Fields"). The resultant bloodshed is believed to have taken the lives of 1.7 - 2.3 million Cambodians, from a population of 7 million. In terms of percentage of population, the reign of the Khmer Rouge resulted in the greatest single act of genocide in modern history.  
In North Korea, between 1994 and 1998, an estimated 2-3 million people have died as a result of famine. The famine appears to be the result of the near total economic collapse of North Korea. North Korean leaders have blamed the famine on flood and drought, although no independent investigation of the famine has been allowed by the reclusive government. 
Freedom and Democracy
Although many Communist nations are named as democratic republics, such as the former German Democratic Republic (East Germany), the Democratic Republic of Vietnam (North Vietnam) and the Democratic People's Republic of Korea (North Korea), they have almost universally resorted to dictatorship or a strong central authority. The Soviet Union, North Korea, Cuba, China and Vietnam, among other nations, have identified themselves as socialist and have fallen, either immediately or eventually, into dictatorship. 
- Output performance under central planning: a model of poor incentives.
- Famine - Genocide in the Ukraine, 1932-1933.
- Case Studies - Chinese Famine, 1958-1961.
- Chinese Famine of 1958-1961.
- From Sideshow to Genocide - the Khmer Rouge Years.
- The Khmer Rouge.
- Peace Pledge Union Information - Genocide Cambodia.
- The Politics of Famine in North Korea.
- North Korea's Kim-made famine.
- Communist Countries, Past and Present.
- Communism in the Soviet Union.