The USSR was a bourgeois state, though deformed due to its unique historical role and position, where the original bourgeoisie and state was demolished by the mass action of the working-class, in alliance with the peasantry, but the subsequent economic collapse of the country, the isolation of the revolution, and the extremely poor solution to the political crisis led to the consolidation of a ruling stratum which quickly became transformed by their role in managing commodity production by alienated labor. The revolution began the most basic of tasks, but then stalled and failed, which left a military-political fighting machine in dictatorial control of the state and the forces of production, but with no recourse but to manage the separation between the laborers and the condition of labor (capital). The nature of this regime and social formation was highly atypical, owing to its evolutionary origins, but it progressively came to integrate, compromise, and recompose itself in world capital's image (its offspring in China taking this process smoothly to its logical conclusion, rather than imploding due to political centrifugal tendencies in the case of the original in 1991).
I'd argue this process began as early as October-November 1917, and there were even hints before, was consolidated by the end of 1918, and the nails in the coffin were nailed tight by 1921. However, the total maturation of the Soviet social formation took decades, and the Stalinist rise to power, Great Purges, and the 'revolution from above' were equally important events, which arguably changed history in of themselves. The final product was arguably not reached until after World War II.
The other regimes with model states set up in replica of the mature Stalinist (and later, post-Stalinist) polity and economy, but were nearly without exception had less vitality and stability than the original formation. All were either imposed by consolidation top-down by military and political forces or blossomed in the wreckage of failed attempts by lagging and backwards nations to modernize conventionally by the 20th c. The "production for production's sake" forced by the state at the direction of the center, combined with some social contract of services and employment, was seen by backward national elites as an attractive option for accomplishing what a conventional and decentralized integration into the world market had not accomplished.
I recommend Daum's The Life and Death of Stalinism, by the LRP, Aufheben's state capitalism theory, Bordiga's work on the USSR, and Paresh Chattopadhyay's The Marxian Concept of Capital and the Soviet Experience.