Communism is absolutely devastating if you've managed to conquer a good to large-sized empire (say, your entire starting continent). If you combine it with the FP, SPHQ, and Courthouses here and there, and toss in a couple of policemen, it's a production powerhouse. It's true that you can't rush without killing pop, but you get such massive troop allotments and you can conscript enough each turn that it shouldn't matter - you're the freight train, once you build up enough production momentum, you're just going to overwhelm everybody.
Cash rushing isn't terribly effective for combat units in the entire Civ3 line in my experience either, everything's terribly expensive until the last few turns unless you have huge coffers.
Plus, since you don't need cash for rushing (only upgrading) you can jack up Science and Lux as much as needed to stay competitive. Communism is a very powerful tool for the warmonger - in C3C at least, in the others it's garbage.