From The Rebel Culture, edited by Robert S. Gold, a bit from Andrew Kopkind:
If there is a revolutionary program in anyone's head, it involves actions along all kinds of fronts to soften the system - by blowing kids' minds, frightening the comfortable classes, organizing the oppressed, questioning the legitimacy of everything. But that would be only a preliminary stage; plans and programs would have to grow out of experience. The Declaration of Independence detailed tyranny by presented no formula for constitutional democracy. Marx carved up capitalism but laid out only the foggiest conception of a communist state.
This last regarding Marx I think is a big part of the difficulty in selling Marxism. With no real plan for a communist state (whatever that is), people have only the failed "communist" states for reference.
(Marx's idea was that communism is not achieved until the state has withered away, so a "communist state" is actually not possible...)
I like the part about blowing kids' minds. I'm lucky to be able to occasionally achieve a little of that in my work, I think.