[n42]

you're talking about three different things: NixOS, Home Manager, and Impermanence. or put another way: reproducible root filesystem, reproducible home folder, automatically reproducing things on boot.

if you want a reproducible root filesystem, use NixOS. if you want to reproduce your root filesystem on boot, use NixOS with impermanence.

if you want a reproducible home folder, use home manager. if you want to reproduce your home folder on boot, use home manager with impermanence.

I only use impermanence on servers. I don't think there's any point using it on a desktop, or with Home Manager, other than street cred. it just complicates things. I don't really even recommend Home Manager for first time NixOS users for the same reason.

edit: and I want to address this comment:

> I'm not sure I'd want to always download some programs that are quite huge

this isn't how it works; the binaries, as well as the entire initial state of the filesystem, are cached in the Nix cache, a read-only filesystem. you're not downloading everything on every boot.