I played around with Rollyo during the Web 2.0 workshop I took last year. I remember it being a cool concept, but I'm just not sure how often I would use it (and to be honest, I haven't used it since!). I attempted to use it during that workshop to gather information for my (then) upcoming San Diego trip by adding links to a bunch of information on the city, some attractions, visitors guides, the hotel's website, etc.
Rollyo could be pretty neat, since it allows you to search just the sites you want for information you need, but I almost think I would rather just save the sites to a favorites menu and go through them one-by-one than search them all together. Rollyo can be useful if you're looking for a particular item or specific information but you're not sure which site it might belong to -- if you've included it in the Rollyo search engine you've built, it would be easy to search for it and find it. But, a downfall of this would be if you have too many sites saved to the Rollyo search engine, you might have to still sort through pages of search results to find the exact information you're looking for.
A use for it within the library might be to create a "search engine" to just look through favorite sites for programs, storytimes, fingerplays, etc. It might also be useful for public computers for saving things like popular news sites, homework help sites, job search sites, etc., for the public to use. It would save them lots of time and would only search sites preselected by library staff for information.
It was fun to try out the program, but I think I'll stick with my favorites menu for now.