I participate in development of the cross-platform indie video game written in C++ called Insatia [1]. We switched our CI server to building almost all the binaries using Nix. With Nix we have proper Clang with libc++/etc for latest C++ features, cross-compiling Clang (for building Windows binaries from Linux), static linking of any 3rd party libraries (useful for portable builds), Wine-based environment for building Xbox binaries (sounds crazy, but yes, it is faster and more stable in Linux than in Windows VM), and all sorts of hacks and tricks, all laid out in .nix files in a quite comprehensible, self-documented way. Now, given a clean Linux machine with only Nix installed and our game repo checked out, I can run a single nix-build command to build our game for Windows, Linux and Xbox. It will take quite a bit of time (hours) and disk space (tens of Gb), because it builds custom compilers and temporary VMs, but it will be cached in Nix store and only has to be done once.
Before that I used giant Docker image with all sorts of build tools installed/built, plus persistent VMs which had to be maintained manually, and Nix made it so much easier. Nixpkgs provides a good foundation - it has almost all software you may need and all the utilities to modify it or add new software. Need to patch or switch to custom version some obscure dependency of a compiler building another compiler building a library you use? Override a derivation, and all the dependent stuff will be rebuilt automatically. For building software Nix can be seen as a kind of super-Docker - Nix store allows for more granular caching than just layers, so incremental improvements can be done much faster. Essentially Nix turns files and packages into values in a programming language, so instead of hacky bash scripts trying to imperatively maintain a file dump, you just compose immutable packages by writing expressions.
That said, Nix is really hard to understand at first, comparable to Haskell/monad tutorials. I remember I made a few unsuccessful attempts at it over a ~6 month period, every time becoming more desperate, and then it finally clicked after careful reading of Nix pills [2] for a few consecutive days. To me, the most interesting thing to discover was that while Nix/nixpkgs do necessarily use some "hard" concepts like fixed point, it is in fact quite "old-school" and mostly about Unix, executables, linking, string templating, contains a lot of bash scripts, etc, so it's not really another Haskell. In fact, traditional building of C/C++ software with autotools is supported in Nixpkgs better than building modern stuff like Rust or Go, due to reliance of the latter on own package managers.
[1] https://insatia.kozinaka.com/
[2] https://nixos.org/guides/nix-pills/index.html