Ranked in these QuestionsQuestion Ranking
Pro State of the art package manager
Atomic non-destructive upgrades / rollback of a system upgrade / declarative reproducible system configuration / unprivileged installation of packages / transparent source or binary deployment.
Packages don't break after a NixOS upgrade as they are prone to with other distros (especially Arch).
Pro Reproducible System
NixOS is configured using the Nix package manager, allowing your system to be replicated and kept in sync across multiple machines. Great for keeping a laptop and desktop in sync.
You can start with a minimal environment and add packages and software to suit your needs as you go along.