I played a game recently which nicely illustrated the advantages and disadvantages of communism and democracy.
I was the Chinese on a large continent with the Persians and French. The French were slightly smaller and the Persians were about half my size. There were three other civilizations on another continent and we were all at peace.
My current form of government was democracy and I had a slight technology lead on world. I had a large military, far superior to the Persians and about equal the French.
At that point, I estimated I could take the entire continent with a sustained effort and reasoned that communism would best suite this. I switched to communism and began my glorious campaign.
Right off, communism was disappointing. I needed to build police stations and courthouses in my main cities to get their production to a reasonable level. Although communism supported my entire military for free and I could reduce luxuries to near zero, those savings were more than eaten up by the reduction in income.
However, once the conquest ball got rolling, the advantages were nice. I had a large amount of captured workers that were very useful for population rushing those first few improvements in my conquered cites. The war lasted quite long and war weariness was never a problem.
Once I polished off the Persians and moved on to the French I had used up all my 'surplus' workers. Fortunately, the French cities were large and provided ample population to sacrifice for those key improvements.
Towards the end of the conquest, the lack of income became problematic. The cost of all of those new police stations and courthouses plus the cost of all the improvements in the new cities really started to add up. The only thing keeping me from bankruptcy was the gold my troops were pillaging from the conquered French cities.
The French had brought all other civilizations in the world against me so trading with anyone was impossible. By the end of my conquest I had lost my technology lead and was broke. Perhaps this was a diplomatic error on my part. I ignored the other continent since they were not a threat militarily. Although they never were a threat, I could have used the extra income from selling resources or technology.
Once the French were defeated, I switched back to democracy and was able to make peace with the other civilizations shortly thereafter. The increased size of my empire put me in a position to easily win the game.
Communism proved to be a real advantage in a long late game military campaign. This campaign left me in a position to win any type of victory. Had I remained in democracy, the campaign would have gone on in fits and starts and dragged out much longer. Cities flipping back would have been a real problem and my new cities would have had few improvements in them.
On the other hand, once the continent was taken, democracy was a real advantage. With all the added cities, income was sky high, which translated into easily regaining the technology lead. I could sit back and win by culture (big help from all those new temples and libraries), win the space race, or plan a monstrous invasion of the other continent.