As a lover of Scheme, that's a big win for me. I really don't want to have to learn a special-purpose language just for package management.
The laziness allows for some really cool and clean patterns that work intuitively and simply while still enabling powerful features. Also the syntax is tuned for clean hierarchical config files. And in some cases being pure helps you not shoot yourself in the foot with determinism.
I still would probably prefer Guile, but Nix is nice enough.
Mainly I just like the much larger ecosystem Nix/NixOS has. I can forsee myself being productive with NixOS as a desktop OS, not so much for GuixSD.
I think of it this way: one new somewhat-fancy config language to learn is much better than the 100 simple config languages you learn to tweak Arch Linux.
This is in addition to whole-system containers: http://www.gnu.org/software/guix/manual/html_node/Invoking-g... .
There's even support coming in Nix for building standard layered Docker containers in a fully deterministic way: https://github.com/NixOS/nixpkgs/pull/11156
The deterministic properties of Guix and NixOS allow some amazing things. I personally prefer Nix as it has a more complete and mature ecosystem, but everyone should check out both projects though.