My description doesn't nearly do it justice - it's a very cool UI - sort of virtual reality meets slideshows and albums, with zooming, 3D flying etc. Here's a tour.
Live labs architect Blaise Aguera y Arcas likens the potential to Narnia worm holes, where you can use the pattern matching and UI to go from one of your photos of the basilica to more photos of the basilica taken by others.
It looks slick - I am very curious to see how they plan to integrate photos from outside photosynth. Wonder whether the pattern matching works at scale at all the various places in the world. They definitely should include geotagging and maps also.
Oh yeah - if you want to see the Basilica, we have a few (44,214) photos from Webshots here