I like the toolbar that comes with Delicious. It makes it very handy to visit sites and to find the bookmarks you want quickly and easily. It's also nice to be able to access favorites wherever you are.
I like the way you have a choice of order: whether most visited or recently bookmarked. It's handy to be able to toggle between these, and I like the scroll down list at the end of the toolbar. Very handy. It makes getting to the sites I use much more efficient.
I'm not sure about the social networking aspects of the bookmarking yet. I've clicked on my Delicious entries' numbers to find others who bookmark the same pages. It's interesting to see what else they've marked. I was able to discover a couple of friends, even though they don't use their real names, through their interests.
I'm not sure how to use this in a library setting yet, except to have helpful sites handy.