The Purely Functional Linux Distribution
Ranked in these QuestionsQuestion Ranking
Pro Upgrading the system is extremely reliable
Since NixOS stores all its packages in isolation from each other in
/nix/store and because of the declarative configuration model, upgrading NoxOS systems is extremely reliable. Furthermore, it gives you the ability to roll back upgrades.
Pro Extremely reproducible state of installation
Every package in your system is generated from a configuration file. This makes it very easy to reproduce that environment. Just copy the config file into a new machine and it's done.
Pro Versatile snapshot system
You can use and test snapshots without rebooting. Booting into snapshots or test configurations is possible without risking the system's stability.
Pro Great for Haskell development
It has all of hackage in it's package manager (which is confusingly named "nix" as well) due to being based around hashing and allowing you to compile in a sort of virtual machine (really just changing the PATH variable temporarily) it solves many of the versioning problems that you commonly have with cabal. Here's a tutorial (there's many others as well) http://www.cse.chalmers.se/~bernardy/nix.html.
Also I should note that you can use the package manager a la carte on Mac and most any linux distro.
Pro Has docker like system built in
Pro Allows parallel configurations for multiple projects
As everything is isolated, you can have on the same machine multiple configurations to meet project requirements that would be mutually exclusive on other OSes.
Con The configuration language is hard to figure out
For good reason. It's a purely functional language. However not even close to bash.