Being using it for almost 2 years. Was very weird at the beginning because of the "declarative" approach they used. But once you get used to it.. Its a life changer.
I've been using it for around a year and really like it so far. It is however very different from almost every other linux distro, so I would think carefully about it before switching. If you're not prepared to invest significant time and/or don't really care about the advantages of NixOS, you should stay away from it.
Pros of NixOS:
Declarative configuration: This is probably the main selling point. The whole system configuration and installed packages are neatly in one place. Using home-manager, this can also replace config files for many programs. All of this is especially useful if you share that configuration between multiple devices.
System rollbacks: If something breaks, simply boot into the previous generation.
Very customizable system: You can freely choose your desktop environment & basic system packages.
Pros of Nix in general (you don't need to install NixOS for this):
Huge package repository (also very up-to-date if you want to use the unstable channel)
Consistent developer environments that can easily be shared
Cons of Nix & NixOS:
Very steep learning curve: You essentially have to learn (the basics of) the Nix programming language.
There are often many ways to do things without any clear recommendation: Channels / Flakes, whether nix-env should be used, etc.
The documentation isn't always great (although it is improving)
If something is not packaged in nixpkgs, it can be difficult to run it, since NixOS doesn't follow the Filesystem Hierarchy Standard. There are some tools to run flatpaks, appimages and arbitrary executables, but especially the later might not always work out of the box.
I hear you with the "many ways to do things". The flexibility in the language to get from input to outputs means every GIT I look at has differently constructed flakes. Finally, after pouring some time in these last couple of weeks i am grasping the basics. Zero to Linux https://zero-to-nix.com/ was the first time it came together for me.
I loved using NixOS with flakes, home-manager and custom minimalistic setup with XMonad or Sway. I was also using Nix with direnv whenever I could for my development projects. At the same time I've noticed that a lot of my programming focus (and time) was being used by solving niche issues with Nix, packaging things for Nix, suddenly breaking applications (because of upstream changes within nixpkgs). I've also had issues with using other ways of providing development environment (npm, pip stuff etc).
I was using unstable channel so breakages part is most likely on me. But apart from that I'm left with the feeling that NixOS is a huge time sink which isn't necessarily worth it if you aren't managing a whole fleet of machines.
Nowadays I am happy not using Nix at all. My development needs are fullfilled by what is available via dnf (Fedora package manager) and npm. I also use podman containers and flatpak a lot more since I've switched to Silverblue.
I feel like nix could be a good 'project' distro, but I would hesitate on installing it on my main computer until I was quite familiar with it and had a few months with it on another computer.
I've played with it an a few occasions, and I like what they're going for. However, I just couldn't put it onto a production computer.
Right or wrong, I feel like I'd have to over-write a LOT of muscle-memory from using regular distros for the past 8 years to use it regularly. If I'd been working in dev-ops, I may feel differently.
I like Nix and used it for a year or so before kind of just getting tired of using a non-standard distro. Mostly just felt like it was trying to solve a bunch of problems that I never had.
Seems like it's targeting devops, only I can't really picture a devops situation where the powers that be would be interested in Nix...