I've been working with Del.icio.us for a few days and I think I finally have a basic understanding of its purpose and how it works.
I can see the value of using this site for bookmarking, in the similar way that Rollyo is used, but with less work. I set up my own login and I like how you can just type in your URL (http://del.icio.us/allnewtechie), and boom! you're there. No password necessary, etc.
Maybe this site would be useful if one is exploring a topic and wanted to see what other people think are interesting sites related to your topic. This would be good for 'lite' research, but not for any topic one would need to explore in depth.