It was up last night for awhile, and I was able to sign up for it and view other people's posts. They rate how "synthy" the set of photos is... (if they all link up somehow, I think its 100% synthy.)
The way it works is you install a small application (you need a windows live id), choose the images, and the synth takes place on your computer. But currently it can only be viewed after its been uploaded to their site. Funny, but there's an awful lot of sets of uploads that are 0% synthy or very very low; looks like people are just uploading completely random sets of images, and using it like a typical photo-sharing site... needlessly adding to the volume of traffic on the site. Perhaps Microsoft should restrict the uploading unless sets are some minimum percent "synthy."