Del.icio.us is more valuable than an ordinary bookmarking site because it includes the social component as well. When I thought of "social" bookmarking I thought it would be something similar to Facebook, but it's not like that at all. It's really just a way to see which way the wind is blowing on certain topics and issues.
Those doing research could also be led to important sources by exploring the tags. I like the idea of searching the tags to find links for student assignments. I searched the tags and found a link to a library curriculum website that seems like it will be valuable to me. However, I don't think I would personally use Del.icio.us for bookmarking. I am used to having my bookmarks in my computer (not on the Web). It takes getting used to the idea of being mobile and having access to all your own information from a computer that's not yours.