Very impressive and very useful for certain Emacs workflows. Also, something tells me the creator of this project and its users won't feel the need to spam HN and r/programming with regular updates on the progress of the project in an attempt to raise the profile of the language it was written in--unlike what we saw with a certain other tiling window manager a few years back.
In fact, there is already a Lisp (Common, not Emacs) tiling WM called StumpWM.
If you haven't heard of it yet, it's partly because the Lisp community isn't so embarrassingly insecure and desperate for anything they can pass off as a "killer app" (see Macsyma or Emacs itself) that they feel compelled to shove it down everyone's throat.