At LogicBlox (http://www.logicblox.com/), we use it to deploy our customers' applications both for testing and production. We also use hydra (http://hydra.nixos.org/) to run our buildfarm. We're soon going to use NixOS for our deployments. If it wasn't clear from that, we're fans... :)

I use it on my notebook and it is the only OS I ever booted on it. Works fine for me, but I do like to do strange things with my system. This means that I do value the ability to revert a problematic change and I am OK with configuring things via low-level configuration files.