Google engineers have just added a new functionality to StreetView that will take some shine off Microsoft's Photosynth. Now you have the option to preview user uploaded images from Panoramio in StreetView mode AND navigate those images in Photosynth-like manner (ie. footprints and relative perspective of related images are displayed on the background image as a white, transparent overlay; double-clicking on the overlay will load a new image into the viewer). Try for yourself: go to Google Map and type "Sydney Opera House" in the search box, then select StreetView option and click on "user photos". When you move the mouse around white overlays of related imagery will start to appear on the screen.
It is all still a bit raw but with more photos depicting a given landscape it should be a much better experience. I believe the next step will be an integration of user added images with actual StreetView scenes. The same "double-click zoom on white overlay" functionality is already implemented in StreetView viewer so it may be only a matter of "merging" the two together.
I must admit that Google's implementation is much more closer to the original concept behind Photosynth than the product ultimately delivered by Microsoft. That is, the starting point was to organise millions of photos available on the Internet into a seamless, 3-dimensional view of the world. Yet Microsoft abandoned that path and released Photosynth as a personal image viewer (and a stand alone product for corporate clients). It looks that Google may win yet another battle for the dominance on the Internet.