Would you suggest the Nix Pills series now? I notice it's 2 years old.
As a "common user", I'd say very much yes. It goes quite deep into the internals, and touches underlying ideas and basic mechanisms of Nix, which I don't think are expected to change [ever].
That said, note that it starts to be relevant when you're starting to write Nix expressions (meaning, write something in Nix language). Sometimes also when you want to read them and understand them. Which... is probably quite soon after starting to use Nix/NixOS, if you'll want to tweak some configs or add custom new packages. If you're really, really just doing first steps with Nix/NixOS, and/or just use the default packages with only simple tweaks, I'd say you don't need to read it yet (I personally wouldn't grasp enough basics then to understand WTF the articles are talking about at that point).
I would suggest it, there is not much that has changed about the core of nix/nixpkgs, the nix language is quite stable and most changes have been additions rather than changing existing features.
It's unfortunate that humans can't trust each other enough that saying "You'll like it after a few months" is good enough, but it's also an inevitable fact of life; for starters, the speaker might be mistaken. I don't think NixOS is for everyone, at least not yet.
And it's hard enough to run arbitrary software that, yes, you do need to learn the Nix language. In most cases it's as easy or easier to package the application for NixOS than it is to run it without packaging, which on the one hand is a benefit to the Nix ecosystem, but on the other hand can be a little annoying. There are workarounds...
But the workarounds require some unusual understanding to begin with.
I think one of NixOS' major flaws at the moment is documentation. There's the wiki, but it's both somewhat outdated and very much not comprehensive. As for Nix, it's been best explained in a series of blog posts, and as good as the Nix Pills series is, you have to find it before you can read it. It's also somewhat short on HOWTOs.
Most people don't like to spend hours writing documentation, though.
The existence of NixOS, and work done on it, is a massive positive externality. Even if it doesn't catch on, it's going to inspire half a dozen clones in the next decade or two. Unfortunately I don't see a good way to get more effort put into it, especially the boring bits (such as documentation)...
I suppose this comment is my way of trying to change that.