I’ve tested out the site and it is a little buggy right now. When searching from the main page you sometimes get a blank page, but if you search again from the blank page, searching works. Also, when you search and get a list of results, they don’t tell you how many photos have been found. In addition, if you choose a photo you expect the previous and next buttons to take you to the previous and next photos within your result set, but instead they take you to the previous and next photos within the entire archive, which is irrelevant to your search results. Hopefully they’ll fix these issues quickly.
You can of course search for the names of relatives using the search engine, but keep in mind that you can also search for locations like your ancestral towns. Even if none of the photos are directly relevant to your family, you might still be able to see photographs from the town your family came from, and learn about how your family lived in Europe.
[Update 1/27/2011: A Google blog entry explaining the partnership.]