Which is not always up to date, and exposes all the available options. I found myself often need to dig into nixpkgs and add missing options myself during the time I was trying out NixOS.
And believe me, that wasn't a pleasant experience.
In the end, I just decided to take the plunge and set it up as my main OS after a year of procrastinating around trying it out. I've been using it for about 4 years now, and wouldn't go back to a traditional distro.
And finally, 4 years on, I just set up a new laptop and it was much easier than its ever been before. I just had to copy over my existing nix config, and nix built out the whole configuration and my user environment (with home-manager [0]). It's always been a bit of a pain setting up my profile before this, and maintaining my .emacs.d, etc; now it's all managed.