My favorite example of how idempotent NixOS is has to do with the DE. If you've ever looked at switching from gnome to KDE, or the other way around, most distros suggest to just re-install because each DE leaves so much cruft around and it's so hard to remove everything in a safe manner.
With NixOS, you just change one line in your config, and the DE is cleanly swapped.
War and Peace. Heard so many good things about it. Despite everything, went in not having super high expectations.
The whole book turned out like a reality tv show. All the characters had some petty drama that they blew out of proportion. Hundreds of pages where nothing really happens, people just complain or bad mouth other characters.
I had to stop half way through.