I've actually had a del.icio.us account for longer than I've been doing 23 Things - I adore using it. (I actually have two: one for personal use, one for professional stuff.) You can check out my professional links here.
Why I think it's a great tool:
First, I spent enough years doing manual editing of HTML on webpages to add links I wanted to check regularly, or access from somewhere other than home, to appreciate the touch-of-a-button functions of del.icio.us. I also love that you can share or not share links as appropriate (for example, I could bookmark login pages for sites I post on, if I wanted, and not share them, but still access them quickly as needed.)
But when you add the ways you can group links (through tagging, and then through grouping collections of tags), it becomes even more useful. For example, I've done quite a bit of work on online safety and literacy education: I can pull useful links together, and do a pointer to that specific category for people by just giving them the category URL (useful if they're not necessarily interested in other links I might have, like our reader's advisory/book ordering links.)
As a library:
We haven't made extensive use of del.icio.us for library info, because we have an internal Moodle site (which students are already used to looking at, and where we can use school email and other tools to easily announce major new content.) But I find it very useful both for finding new information, and for keeping track of topic-focused links.
I can also see us potentially using it for research topics (much the same way we currently post weblink lists on our website for student/class use.) However, in that case, there's already a method in place, and changing it can be confusing for both students and teachers.