5 points
I find it useful to not have to remember how I set things up when I last touched it months ago. You can do really ricey tweaks if you want to, without worrying about breaking the whole system, or having to set it all up again if you have to reinstall.
I work in Devops, so being able to track my system in git is insanely useful for maintainability.
The fact that NixOS has fearless bleeding edge is just a plus; Being able to install the latest packages before Arch even gets them, without worrying if something will break.