We've implemented it in a E-learning system used in some Dutch High-schools. A few talks we've given at the local Clojure Meetup:

- https://zeekat.nl/news/2015/02/12/event-sourcing-at-studyflo...

- https://speakerdeck.com/helios/2

- https://speakerdeck.com/helios/event-sourcing-cqrs-and-scala...

Right now the system is running with ~800 million events, with roughly 1.5M added per day :)