The Sierra II is arguably the best SSN the Russians have, with all the features of the Akula class, along with a titanium hull and relocated torpedo tubes (they are amidships, as is the manner with US subs to allow for a large sonar array in the nose as may later model Akulas).

They were dropped by the Soviets in favor of the Akulas because they were too damn expensive.

Truth be told, I’m not sure of the military value now, though one might use it to escort a Russian boomer, and there is always the intelligence value of parking one of these just off the coast of an area of interest and either listening to signals, or inserting operatives.