Yeah I've been running the Arch variant of this on my personal laptop for a while now. Of course no stability guarantees with this for now since you'll be running systemd from source.
mkosi runs pacman for you and then packs up the result as a disk image. It also builds a unified kernel image and does a bunch of signing. The new /usr partition and UKI are then installed with systemd-sysupdate.
Building the next update locally is slow because erofs has to compress the entirety of /usr on my rather old laptop and that takes a while.
Aside from that, I'm using flatpak for firefox and for some reason it takes absolutely forever (like > 15s) to start firefox on my machine, still need to look into why that happens.
Aside from those it's very usable. But of course it's running systemd from source so I'm not going to actually recommend anyone to run this who is not a systemd maintainer until we iron out the kinks.