Guix is niche, but it's very much 'real software'. It does useful things well, and it solves some problems that very few competitors adequately do. It's used for reproducible scientific computing on some HPC clusters, which is a pretty cool niche if you ask me.
The Guix package collection is also sizeable and growing exponentially-- within a few years, it'll probably outgrow most Linux distros in terms of size.
I had to use Nix in a project of mine and was surprised at how clunky it felt compared to guix. Writing packages meant string concatenations and juggling outputs to the command line.
Same here. I used NixOS as my desktop operating system for a few months I think. It did not go great. After hopping a to a few other distros, now I've been using Guix as my OS for a few years.