Recs.
Updated
SpecsUpdate
Pros
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 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.
Cons
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.