Revolutions. Do they help or hinder society? Do the ends always justify the means? If not, then where do you draw the line and under what circumstances would you cross it? Will the world really be a better place if the [insert societal cancer here] are done away with?

I've been thinking about this lately, mainly due to some of the answers given in the latest religion thread (but please don't bring religion into this unless it's relevant, such as in the case of an anti-religion revolution). What are your thoughts on using violence or coercion to "throw off your shackles"? I'm not talking about the Scientific Revolution of the Sexual Revolution here, I'm talking about violent political or social revolutions with the supposed intent to create a better world.

What are your thoughts?