At LumiGuide we use NixOS on our development machines as well as on all our production servers. I used ansible before but NixOS is on a whole different level.

See the following for a story about a system we just deployed which uses NixOS:

https://news.ycombinator.com/item?id=9690683

Yeah, I've been running NixOS on production servers for about a year. I'm at a small startup, so it's not huge scale (yet!) but it works great.

Got any problems with package availability?

I imagine those should be very easy to fix, is that true at daily use?

All the packages I need have been part of nixpkgs, except for node packages from npm. For those, I run npm2nix to generate nix expressions from the package.json files. That's occasionally tricky when npm packages do "clever" things that don't work when translated to nix. Generally, though, npm2nix works pretty well.

I also have about 3K lines of nix code for packaging my apps, configuration, cluster definitions for nixops etc. There's a bit of a learning curve, but I find it really pleasant, now that I have the hang of it.

I've been using it on my HTPC for a while. Getting it to work for a relatively exotic setup (boot into Kodi, still allow admin logins, mount NFS media shares on boot) was a bit fiddly, but once it worked it's been great. The one concern I have is that it was pretty happy to recompile Chromium all the time, but switching to Firefox made it more bearable.