Very nice.  The graphics you took the time to do the "Alice's Restaurant" thing to really make this a very clear article on what Rollups and Cubes actually do.  The graphics clearly demonstrate why you might want to use the Grouping function of a Rollup or Cube.

There's lot's of folks who have no idea what a "subtree cost" is, how to determine it, or what unit of measurement it is.  Also, although very obvious from the graphics, simple example Selects for the isolation of subtotals and grandtotals would be very helpful for newbies.  A very brief explanation of these two things would have made this a "six star" article.  

Outstanding article overall!  Thanks for sharing it with us.