I think that for this layout to be truly useful the underlying tree has to have particular structure (not too deep an very wide), things like music library or S

I think that for this layout to be truly useful the underlying tree has to have particular structure (not too deep an very wide), things like music library or Smalltalk Browser matches this well (mostly because the depth is inherently fixed), while arbitrary tree structures (think Unix filesystem) does not. Another case where this kind of view is very useful is when the user only has vague idea where in the immense hiarachy the thing that she searches for actually is (think online component catalog, with Farnell's eshop having quite similar but by default hidden navigation tool)