Yeller is an exception tracking service whose goal is to provide clear, actionable information to developers to take the pain out of examining application errors. They chose Clojure and Datomic as the platform for Yeller in order to make the development process more effective and more efficient.
Clojure and the JVM specifically helped them create a robust runtime service handling spikes of millions of events per second.
You can read all about their reasons for choosing Clojure and Datomic on their blog. You can also watch Tom speak about their decision at EuroClojure 2015.