I also really like flakes. I know they are still experimental today - though hopefully that'll improve with this RFC - but they make it trivial to have an out of tree package. For example I just recently created a flake for a vendor OpenOCD fork [0] - and now I can reference that in my home-manager config or from some other project.

[0] - https://github.com/benpye/openocd/blob/mrs-2023-08-07/defaul...