With my #Guile hat on, I can tell you that being able to test complex setups in a single command is pretty cool. Example:
guix build guile-next --with-git-url=guile-next=$HOME/src/guile \
--with-branch=guile-next=main --target=x86_64-w64-mingw32
Those #Guix folks are doing a great job. 😇