I ran Gentoo for a few years, but the distros I used most in the years immediately before discovering NixOS were (based on) openSUSE, Arch, and Ubuntu.
NixOS is more similar, culturally (rather than technically), to Debian than it is to Ubuntu or Fedora, both in the sense that it's not backed by a large corporation and also in the sense that it doesn't promote a particular vision of the Linux desktop.
On a technical level, its package management approach will feel like a larger paradigmatic leap, coming from a Fedora or an Ubuntu, than it would from a Gentoo. NixOS doesn't have quite the same notion of a repository as either of the former. The concept of 'overlays', used to extend Nixpkgs, would be familiar to a Gentoo user but not a Fedora user.
That said, an Ubuntu user won't notice a lot of compilation from source, and in fact they'll likely find Nix pretty fast as a package manager. And likewise, NixOS is more different from any of those than it is similar to Gentoo.
An Ubuntu user switching to NixOS will likely be
- impressed with the wide selection of packages
- happy with how easy it is to iterate on a config
- delighted by the rollbacks
- pleased with how easy it is to add most software to Nixpkgs
- eventually frustrated by the inflexibility of having to package things for Nix to get them to work well on NixOS
- appreciative that their system doesn't accrue cruft over time
- perplexed with having to learn slightly different processes for packaging things in different language ecosystems in Nixpkgs
- blown away by the knowledgeable and helpful community
depending on how deep they go with NixOS. Overall, I can't say if you'll like the whole declarative paradigm or not, which will be the biggest difference. But bear in mind that you can enjoy many of the benefits of Nix while still keeping one foot 'in both worlds', by using Nix on top of Ubuntu or Fedora rather than jumping in with NixOS.
Jumping right in with NixOS is a good idea if
- your needs are simple, or
- you have a bit of packaging experience (for any distro), or
- you have an interest in functional programming, or
- you like learning new things, or
- the reproducibility and uniform management of your system's configuration is highly appealing to you.
If you have less Linux experience, or you feel less inclined to commit to the whole paradigm for any reason, try giving yourself a taste of Nix and NixOS' declarative style by running Nix and Home Manager on top of your existing distro of choice! I think NixOS is in a mature enough state that there's just a ton of functionality and packages in it, and lots of Linux users could be happy with it as-is. But for those who are thinking about getting into extending it and molding it to fit their own needs but aren't sure about the hairy details of that kind of work, running Nix on foreign Linux can be a good, noncommittal way to get a sense of what that work looks like.
On the whole, I think any experienced Linux user will at least find it interesting. But the more common response from those who really get a handle of it is 'I could never go back'.