The ability to link features by using hyperlinks is great.
GPS cameras have made taking photos with geographic location and orientation very easy. Making a point feature with this information, as well as a hyperlink to the photo file location is a great way to create an interactive map. Clicking on a point can bring up the photo - just like the Panoramo feature on Google Maps.
However - you have to have a copy of the ESRI software to use this feature. I think it would be a great tool to be able to export a map to PDF format and have the PDF retain these hyperlinks. This would allow you to send a PDF maps with a folder of photos to a client - and they could use the map to view the photos quite easily.
Making a mapbook with an overview map, with hyperlinks to the inset maps within the same PDF would also be useful. The PDF would appear to be an interactive map - clicking on areas to zoom in, with 'click here to go back to the overview map' links on each inset map.