We use NixOS in production in our product, for our cloud services, and for some development environments.

We build everything with nix from sbt projects, go projects, haskell projects, purescript projects, docker images, and npm projects.

It's an incredible tool.

NixOps is also a good tool but there are a lot of issues with it we haven't had time to contribute fixes for. We also wanted it to be lighter weight and to decouple system specification from system deployment. So I wrote a tool that handles performing the same system deployment operations that NixOps does for you and you can simply pipe the nix path result from nix-build to it. We use terraform to specify and manage the cloud resources.