I have just used Fascism for the first time on CTC. 70% water, monarch, continents, I am the Mayans, I just exterminated the Egytians and now own my continent with over 100 cities, and so i thought I'd choose a government that reflected my Civs opinion of the other civilizations!
After a short rev, I pooed myself to see me earning -500gc a turn, whereas I was earning +200 and 70% science. But after the assimilation of the population into my empire fascism really picked up. WW is a nightmare for a nasty civ like mine, and I ended up with higher production from my civ, but with less science.
In retrospect, communism would have been the best choice. But I wanted all the other Civs to know "you exist, because I'm letting you...for now". Nothing says that like Fascism. And now I have an entire continent of worked tiles and over 80 workers sitting around doing nothing, and an invasion force of around 100 infantry with a small amount of artillery razing their way across the other continent..actually, that sounds like Communism?!
In reality, Republic is the best gov when your at a level that matches your ability. If you can use Democrazy, then you've got it too easy. If you need communism,then your empire is too big and thus you're clearly going to win.