Therein lies a fundamental disagreement among socialists. Anyone to the left of Stalin - who rolled back pretty much all of the social reforms that came about under Lenin - believes that the USSR was damned from the start. His centralization of power, and the failure of the Bolsheviks to block such things from occurring, proves to many the inherent problems with Marxism-Leninism. You'd be genuinely hard pressed to find anyone nowadays who looks at the USSR and says "Yes, that is what a socialist country should look like," because it was a complete failure in many regards, and where it succeeded was not worth the death toll. The most basic tenent of socialism - the "dictatorship of the proletariat," or worker-controlled government by and for the people - was replaced with an oligarchic dictatorship of party insiders.

Remember, Stalin killed everyone he disagreed with. Socialists included. I'm a Left Libertarian/Libertarian Socialist, and I'd of been on the firing squad with the anarchists, trotskyists, and social democrats. But the actions of the USSR, who followed a specific branch of socialist thought, can hardly be used as the be-all end-all argument against the ideology.