> Is either #NixOS or #GUIX usable as an OS for day-to-day use (coding, browsing, occasional gaming)?
Yeah, definitively. I use only NixOS on all my machines for about 3 years already. My machines are working so well nowadays that I rarely have to change anything [1], even during major upgrades (between stable versions).
I don't fear anymore if a machine could break tomorrow. I know that it will be very easy to restore it to a working state. Last time my working notebook broke I didn't even bother making a backup (except from some data on my Home), just installed my config and it worked exactly it was working before. Also, I don't fear changing my system: if something brokes after the changes, I can just rollback.
I use for all those three purposes, mainly coding and browsing, but some light gaming. Steam actually works better on NixOS than any other distro I tested before thanks to the isolation that Nix provides.
> a question on dev environment isolation. [...] Or is it better to just use their respective package managers on top of regular Linux?
I imagine those are part of the same question. Generally what I recommend for newbies is to start small: yeah, it is fine to continue using your respective package manager, specially if you don't need "native compilation" [2].
After you get more experience with Nix though, I would recommend you to start isolating your development environments. Nix has its own `nix-shell`, that it is kinda bare-bones but works well, and can be very practical once you use something like `nix-direnv` (https://github.com/nix-community/nix-direnv/). There is also this `devshell` (https://numtide.github.io/devshell/intro.html) project that is interesting.
> Also, anyone here doing desktop #virtualization? Is there a sense of running a #VM per project? Is that a viable alternative to dual-booing Windows when I want to do some Windows dev or play a game?
I put those two questions together because yes, I ran desktop virtualization on NixOS, and yes, I ran Windows on a VM to play games thanks to PCI-e passthrough (my dedicated GPU is connected directly to the Windows VM so the performance is near native) [3].
Again, I will not recommend you do to this right after starting NixOS since it is one more thing to possible frustrate you, but just so you know that it works and works well, specially because my VM configuration on NixOS is also on my `/etc/configuration.nix` and it is fully reproducible. Coincidentally I was messing with my VM settings yesterday and "broke" my VM multiple times, however I just run one command and restored it to the previous state.
However I never bothered to dual-boot NixOS/Windows, because I barely use Windows nowadays so I find rebooting my system just to use Windows bothersome. Probably not your case, so maybe dual-boot makes more sense. This article should help and seems very easy to do (again, disclaimer, not tested because I never needed it): https://nixos.wiki/wiki/Dual_Booting_NixOS_and_Windows.
About the VM per project, unless you want to do this for some other reason (security maybe?), you don't need to. As I said before, you can use things like `nix-shell` and this will already give isolation enough for development purposes.
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Some closing words: remember, start small! Nix/NixOS is a journey. You will learn eventually what you need. However, if you try too many things at the same time you will feel frustrated. The official documentation is good, however it kinda assumes that you know what we are talking about already, and it is also very big and not well separated in steps, so it feels overwhelming for newbies. To supplant this there are many blog posts, however some of them are out-of-date (it shouldn't be too much of an issue though, since Nix tries really hard to backwards compatible).
You will eventually need to learn the minimum of the language to read the source code. It is less difficult than it seems, because while Nix as a language is strange at the first glance, it is a very well designed DSL for the domain. After sometime editing your own configuration you will naturally learn the Nix language, and can start to understand what is happening.
For where to start, I recommend trying to replicate your current system in a NixOS install. If you already use another Linux distro try to get the same desktop environment and packages that you already use. If you're coming from Windows/macOS try Gnome or KDE. Just tweak the system until you're happy with the result. Eventually try Home-Manager to manage your Home configuration in the same way as in NixOS (fully declarative), and once you feel happy, you can try Flakes (making you configuration fully reproducible). This process can takes months or even years, but don't bother or rush, just continue your journey. It really pays off in the long run.
[1]: well, I am on a "coding spree" right now so I am doing many changes, most to refactor some things that I wasn't happy before; but before this coding spree my configuration didn't change significantly for almost a year.
[2]: what I call having to compile C/C++ libraries in an another language like Python (giving it an example here, almost any language has a similar mechanism since C/C++ ecosystem is huge), that generally needs to dynamically link with whatever you have in your system installed. This is possible on Nix/NixOS, and easy once you know what you're doing, but it is different from anything else so it does trip newbies.
[3]: if you're interested on this approach, I used this guide when started: https://forum.level1techs.com/t/nixos-vfio-pcie-passthrough/.... To get it fully reproducible it though, there is no way on NixOS itself yet, however there is a good guide here: https://nixos.wiki/wiki/NixOps/Virtualization#Setting_Up_the...