I've used it before, and run a nixos server at the moment.
I think guix is a lot more ergonomic for package definitions and system configuration than nix, I also think the documentation is a lot easier to break into + it has a semi graphical installer.
the big downside being it doesnt come with proprietary drivers which is pretty annoying, but can be worked around with nonguix -- https://gitlab.com/nonguix/nonguix which also includes a few other proprietary programs, if thats something you need
I have not used GuixSD, but I peek evey now and then on its progress.
Like the sibling comment says, it has less warts than Nix and Nix language, but depending on your stance on Free software, you may need nonfree repo.
Additionally, NixOS has far more mindshare and manpower and has a vast repository, something to keep in m mind if you ever want some exotic piece, Nixpkgs has higher chance of having it already packaged.
I think guix is a lot more ergonomic for package definitions and system configuration than nix, I also think the documentation is a lot easier to break into + it has a semi graphical installer.
the big downside being it doesnt come with proprietary drivers which is pretty annoying, but can be worked around with nonguix -- https://gitlab.com/nonguix/nonguix which also includes a few other proprietary programs, if thats something you need