The best answer to that question is "it depends." Communism can be a great government if you have a few cities widely spaced out over several continents. However, if you have significantly more than the optimum number of cities (12 tiny, 16 small, 24 standard, 36 large, 48 huge I believe) then communism is not what you want. It will then cause massive corruption in your core cities while giving you 2 shields instead of 1 in tiny colony cities. If you have 100+ cities... don't even think about it.
I personally believe monarchy is much superior to communism as a wartime government, as money and not population can be used to rush projects and a central core of cities will still remain uncorrupted. It is, of course, up to you as what to use...