We're strong adopters of the whole Nix ecosystem[1]. Having a reproducible, deterministic build of our applications and a functional configuration of our prod and dev server was a huge win for us. We can provision an exact-and I mean literally exact-copy of a cluster of machines in AWS with a click of button!
[1] My Ex-coworker, Rob Vermaas talked about it in NixCon 2015: https://www.youtube.com/watch?v=nuyuA43q9NE