Seems neat, better than Terraform, although limited to just nix and a few providers. Hopefully that will grow. I've been working on a provisioning tool myself. Provisioning is hard, and supporting multiple APIs can be really difficult.
So what I do is have all system level tools installed using NixOS: the OS, logging, systemd, Docker, sshd and so on. Then I have my applications built as Docker containers and provision them as systemd units also configured by NixOS files.
There are probably lots of systems running standard Linux packages with more complex configurations (i.e. Mailservers, maybe databases, proxies etc) where I could imagine it being useful? Or maybe their desktop work makes more sense than I thought–my desktop environment is much more customised than any server, and I've never tried if the setup scripts I try to maintain actually work.
I feel NixOS definitely gets some stuff wrong, and I hope at some point it gets some good competition. Mainly because the Nix language looks a bit awkward to me, and also because I don't fully get their package management situation. But the idea of having the operating system be configured from a specification is just a level up from old fashioned linux distros.
It's great to see NixOS and similar tools gaining ground. But personally I find that docker's configurations simpler to grok than NixOS's syntax. Plus it's interesting the granular level of control over system security and system access gaiened via docker (really mostly Linux secomp controls and process namespaces). It's kind of like having capabilities built into the OS on top of specification driven system configuration.
NixOS is capable of creating a Docker image given a Nix specification, by turning the Nix store into an archive. These images are quite minimal since they contain exactly the dependencies needed for your app.
NixOS also has its own container system, so your server configuration can simply specify a subconfiguration that will be spawned as a container. The containers reuse the Nix store from the host system, which means you get a fine-grained caching system.
I haven't used those things much so I don't really know how production-ready they are.
I sometimes use Docker (with Dockerfiles or DockerHub images) on NixOS as a way to run systems that I don't want to package with Nix. I've made a little Nix module for declaring Docker services as systemd services.
That's part of why I don't feel like Nix is a limitation. I have actually used NixOps to deploy NixOS configurations that include Docker services, so my feeling of Nix is that it kind of supervenes on everything else...
Still, Nix should be at least an inspiration for next real world package managers.
If possible I'd prefer to use one tool for both OSes.
Other distros are also affected by these issues, so I think they'll be solved eventually.
Most of the pain is building stemcells (the target disk image). Microsoft haven't quite made their peace with the idea.
https://github.com/NixOS/nixops/pull/634
I cherry-picked this into my local NixOps repo and have been using it for several weeks to deploy to Vultr servers with no issues, except one -- the author has hard-wired an assumption that you're using Btrfs for your root filesystem. I didn't want that so I simply replaced that hard-wired assumption with ext4 in my personal branch. Now everything is working great.