On the one hand, it's great, as so many others here and TFA have attested. Declaratively specifying your system configuration and using snapshots to keep track of everything is a complete game-changer. Similarly great is the absolutely huge universe of installable packages. The coverage here is so much better than what's on offer from Ubuntu or Fedora.
On the other hand, the current implementation is still a bit of a shit-show.
First, there's nix-the-OS and nix-the-package-manager which is pretty confusing. Effectively it means you manage your OS with one declarative system and your local/home config with another. Then there's "Flakes" which I never quite understood, that seem to offer a different modality altogether.
Second, installing packages is nice, but also confusing. Do you install a package or a service? Often both are available and the difference is not always clear. Eventually I learned to choose a service whenever one was available. In either case, the tendency of package maintainers is to install the smallest possible version of whatever you asked for. For example, I wanted KDE but what I got was a bare minimum version with plenty of missing apps and functionality that could only be fixed by adding extra components, one at a time, after debugging whatever was currently breaking.
I appreciated that services and packages can be configured in the configuration file. But the options exposed are usually a partial set of what's available -- without extending the installations scripts yourself. So now my "declarative" config is a mix of what's in my nixOS config file and what's in my manually edited /etc files.
Third, the documentation, mentioned by others, is a mess. There's all kinds of information about old and new versions. The interfaces of the command-line tools seem to have changed between the 25.05 stable that I chose and the then-upcoming 25.11, which made following-along harder than it needed to be.
I eventually gave up because I needed a working machine and not a new hobby. I was left with the impression that NixOS might be a good choice for system admins, but perhaps not yet ready for desktop Linux users.