This is a usability problem that happens often to me:
I open a read-only system file with vim, even editing it, because I'm not attentive enough, or because the vim on the system is badly configured. Once my changes are done, I'm stuck having to write them in a temporary file or losing them, because :w! won't work.
Is there a vim command (:W!!!) that allows you to write the current buffer as a super-user? (Vim would ask for your sudo or su password naturally)