The world has yet to see a working example of either Democracy or Communism. Both are ideals to strive towards, impossible to reach.
Anyway, my opinion on the matter is that communism is flawed, not because each example in the past has become a dictatorship, but because it underestimates the power of a free market economy. Competition drives prices down and innovation up.
I don't believe in Libertarianism though. A mix of capitalism and socialism is necessary to prevent the rich from getting richer while the poor get poorer.