It is quite stunning, if you ask me, however it was very slow, but now - FAST. Previously, the process of stitching these photos together took weeks of processing on specially configured servers. With its latest version, Microsoft has managed to shrink that into around the time it takes to upload your photos. Over a decent broadband connection you'll immediately see large thumbnails that quickly begin to sharpen as data fills in the missing pixels. You can continue to zoom into these areas and they'll sharpen up even more on some of the super high-resolution shots.
You can stop reading now and go try it for yourself. It is pretty neat and Photosynth will take any resolution of photos you throw at it. Not to mention, Microsoft is giving users 20GB of online storage for their Photosynth collections. Being that they are 3D, that big number will get smaller and smaller, but at least you'll have some nice pictures to show for it!