To my knowledge it was pretty one-sided. Although they came up with some interesting solutions to problems (the Soviet space program had many), their overall doctrine (generally quantity) was so dissimilar from American approaches (generally quality) that what was there wasn't really much applicable and worth stealing.

The US did in fact get its hands on (and even regularly flew) a surprising number of Soviet aircraft throughout the Cold War - generally the intelligence value (how to counter one in a dogfight, how good they were getting at working with exotic alloys, how skilled their factory workers were, etc) was more useful than whatever technology was in the plane. There was the discovery the MiG-25 would trash its engines if it ever did the speeds the West thought it regularly did, and the MiG-23, though fast, was actually a deathtrap - there didn't seem to be a whole lot to love.

But there's one example I can suggest though it's not really reverse-engineering: the Soviets were first with the off-boresight missile/helmet mounted sight combination, and (although it wasn't a new idea) how well they had it working was a huge shock when it came to light after the Cold War. This actually allowed, in certain circumstances, a huge degree of superiority over Western fighters. It's only been with the new-ish AIM-9X that the US has gotten anything like it.