The Purely Functional Package Manager.
Ranked in these QuestionsQuestion Ranking
Pro You always and atomically get an isolated consistent profile
Every time you install, delete or change anything you get a new fresh copy of your user environment (set of symlinks to files in /nix/store) that's stored in the same /nix/store and handled mostly the same way. Your "profile" (symlink to one of environments) is updated after everything else is ready, so you'll never end up in a half-finished state of your system.
Pro Uses binary caches (that are provided) so you don't have to build anything locally
Due to its functional nature, it can just download a binary package with the same hash if its available and it'll get the very same package as you'd build locally (to the last bit that is).
Pro Can keep around several profiles for you to use
You can have different (probably overlapping) sets of software installed in two or more profiles that will be handled (changed, versioned, upgraded, reverted) independently. All software will be installed in the same /nix/store, so any overlaps between your sets will be physically installed only once.
Pro Does not require root priviledges
Everything is owned by your user (unless you use more powerful multiuser mode that doesn't require root either).
Pro It's easy to revert any change in your environment
Every time your profile changes, you get a new generation of your profile and older generations are kept around, so you can easily (and atomically) revert to older version of your profile.
Pro Packages installed with Nix don't overlap with packages installed in /usr/local
Since packages are stored in the /nix directory, it won't overlap with anything you'd install in /usr/local or wherever else.
Pro Packages don't change after they are built
Nix treats packages like values in a functional language. Since they are built by functions without side effects they never change after they are built.
Pro Easy to install
Requires just one shell script to install, only one directory is created via sudo. This makes for a pretty simple install method that most can grasp.
Pro Very easy to share compiled binaries between machines
You can share packages via HTTP or SSH (push or pull) with just some simple commands.
Con Cannot handle filetypes that have different semantics across different versions
While the functional approach that Nix takes is great for sandboxing binary artifacts of packages, it seriously lacks any power in handling configuration files or user data. It's difficult to upgrade and downgrade files where semantics and syntax can change between versions. Especially in Debian/Ubuntu it can cause severe problems where the upgrade process blocks and the user needs to resolve the 3-way merge.