As a happy nixos user, I have to say that it's conceptually great, but that right now, the management tools are abysmal. Not only are the command line utilities unintuitive to use and pretty slow, but there is simply no graphical frontend, which means that I'm unable to switch family members to it.
Your complaints have been heard, and there are a lot of efforts to address them :)
There's currently an effort to make the command line utilities easier to use[0]. There's also an effort to bring PackageKit to NixOS, which should make a few GUI package managers work[1].
Most of the "standard" packages are already in it, and those that aren't are fairly easy to add in. I created and maintain the Matrix.org Synapse package, for instance, and it was a breeze to package myself, even without any prior experience of nixos packaging.
