Are Campagnolo and Shimano 11 speed chains interchangeable? I have an Athena 11 speed group set, but can get a good deal on the Shimano 9000 chain. Does anyone know of any reports of issues combining the two?
Leonard Zinn has discussed Shimano and Campy 11-speed drivetrain compatibility several times in response to reader questions.

Technical FAQ: Drivetrain compatibility for 10- and 11-speed

Technical FAQ: Follow-up on 10- and 11-speed compatibility

Drivetrain compatibility hidden in plain sight

Although Zinn doesn't specifically address chain compatibility, I'm not sure how you could successfully mix drivetrain components if the chains were incompatible.