commie scum!
seriously though, communism ...
- causes corruption to be shared equally by all cities, which may be good if you have a smaller empire. i usually have some cities that are way off yonder that i got from war retaliations that i don't care about, so i never want their horrid corruption adding to my productive cities.
- lets you use 4 garrisoned units as police instead of 3, ie if you have 4 ground combat (no cannons planes or boats) units in a city they will make 4 unhappy people content.
- gives you better chances at espionage missions
- gives no war weariness
- kills people to rush projects. this is the big downfall to communism over monarchy. under monarchy you can just pay for stuff, but under communism rushing a project always kills people and then makes that city more unhappy PErMANENTLY.
you can check a chart of all the pros and cons of each gov at
http://www.civfanatics.com/civ3infocenter.shtml#governments
i personally never use communism. i am playing a conquest-type game right now (didn't start out they way but the entire world declared war on me

) and i am still a monarchy in 1830. i make a ton of cash so being able to spend it on rushing is not a problem. i also used a great leader to rush build the suffrage wonder so i'm going to go to democracy after i end the current war with germany.
good luk!