I have to say in support of Ren that I have finished entire games without ever being in Democracy. A well managed Communism can keep up in science and totally kick butt in war. Still, if we can manage the unhappiness, I think the fairly limited campaigns we are currently planning can be successfully achieved in a Democracy, which will always be better than Communism in the science/gold dept. I would say, however, that if we fail to get the UN, a switch to Communism at strategic times would be very attractive in order to sustain an offensive over a few turns and not have the Senate interfering.
As we've all voted in this poll already, I might suggest a new poll for this subject should we plan a large and/or sustained offensive in the future.