Since Stalin was both an early Russian revolutionary and the nadir of "how the Soviet Union turned out", it's hard to accept the reading of history that says the Soviet Union didn't turn out according to plan.

The Bolshevik coup in 1917 pretty much ended democratic socialism in Russia. Prior to that, workers were forming self-organized soviets (essentially a union government) that, at least from my vantage point, looked pretty much exactly like what a communist government is supposed to look like. Lenin didn't think Russia was ready for socialism yet, so the Bolsheviks seized power away from the soviets and formed a dictatorship. I'm not a socialist (well, I probably am according to today's definition), but if I were I think I'd say that's when the Soviet Union stopped being an effective socialist state.

Edit: just to clarify, this is a brief gloss that leaves out a lot of nuance.

Lenin called the new system State Capitalism and talked about "control, supervision, and accounting" as being the solution to the country's problems [1]. Even Marxists warned about the totalitarian nature of Lenin's project. These essays by Chomsky explore the topic: The Soviet Union Versus Socialism [2], Objectivity and Liberal Scholarchip [3]. Chomsky gives Luxembourg and Pannekoek as examples.

[1] http://marxistsfr.org/archive/lenin/works/1917/ichtci/02.htm...

[2] http://www.chomsky.info/articles/1986----.htm

[3] http://www.ditext.com/chomsky/1968.html