This column type is brilliant in theory, but unfortunately they don't appear to work as expected. Here is my assessment of the Summary Links Data column type.
In SharePoint 2010, if I add a Summary Links Data column to document library, then add a new document, the new or edit document properties page looks correct. The Summary Links Data column is represented by an editable summary links control.
However, after the links have been added and the document uploaded, you return to the document properties using the "View Properties" page. The result of this is that the links are not visible. Huh?
OK, so let's try a custom list. Added the column to a custom list. Adding or editing an item results in a non-editable field. So, the Summary Data List story just got even worse in custom lists...