This would solve a major issue with SteamOS. The SteamOS itself is immutable/read only as you know. This means, one cannot install or make changes to the core system (it is possible, but that is temporarily until update will revert those changes back). That is why we have the Flatpak support. But many software are not available there (especially CLI tools, if you use them).
This is where the Nix package system comes into play. In short, it allows for installation of programs without touching the immutable part of SteamOS in a controlled and safely manner. These would persist even on SteamOS system updates. For a better explanation, have a look here: https://nixos.org/guides/how-nix-works.html
Valve confirmed: https://github.com/NixOS/nix/issues/7173#issuecomment-1603858003 that Nix package support will be integrated into SteamOS! That’s huge. In example I have tools which I cannot distribute easily for Steam Deck users, because Flatpak does not support CLI apps. Nix would solve this issue!
I don’t use it as a desktop replacement (I still have a windows desktop). But I do but default have my deck docked and hooked up to a TV, in desktop mode.
I’ve installed various flatpack programs, to include discord, chrome, and a couple jetbrains ides for various languages. They work great. I’ve done a couple non steam games (diablo), with mild annoyance but success in the end.
I have not used any office programs on it (as much as it’s cool to like Linux, I just think Microsoft wins by a lot there, and have no desire to get them working on Linux).
The deck itself has some power limitations, but I have no reason to think that they would translate to steam os on a more powerful machine.
So basically, it probably depends on what you want. It’s pretty reasonable. Definitely good enough for my secondary computer that blocks ads for videos on my tv. But personally, I still want a windows computer, because windows is the default assumption for most software I use.