Must-have? Hardly. It's much easier to run `du -sm | sort -n', both of which are already installed everywhere along with coreutils.

Fair enough; "must-have" is a bit strong. But, when I'm responding to a freespace alert on one of 2000-300 servers I've never seen before, the "visualized" data layout is easier to digest than a list of numbers - plus ncdu can drill-down through a directory tree quite nicely.