This is init, with the exceptional property that you write your scripts in Guile Scheme (an implementation of a standardised Lisp dialect). That should allow for a more robust and debuggable way to write init scripts.

GuixSD is kind of immature at the moment, and I have driver issues with linux-libre, but in the coming weeks I'll have the time to try to run it with a custom kernel that supports my hardware. If I succeed, as an Emacs user it'll become turtles all the way for me, which is very exciting (GuixSD is Guix + Shepherd + GNU + Linux Libre).

"kind of immature" is a bit vague, but I disagree. I'm using GuixSD on almost all of my machines (a server, two laptops, an audio workstation) without problems.

It is quite easy to overwrite the kernel package to use in a system configuration (e.g. to use a kernel with the RT patches applied), but I should say that I use the default kernel on all but one machine.

It's the exact opposite, it's an init system for the GNU project.

Much closer to a GNU/Upstart. Doesn't try to do much more than manage services, from the looks of it.