CircleCI is a Clojure company from day one, so there was no need to force it on people. We've found a lot of advantages from this, not just on the code side but also from recruiting.

One problem we've had is that our frontend isn't clojure/clojurescript (its coffeescript+haml+knockout). In theory that means its easier to find frontend developers and designers. In practice it has meant creating a frontend/backend division where the backenders in particular don't like to work on the frontend. We testing out whether a move to clojurescript (and hence being a truly one-language company) will solve that.