I have been using del.icio.us for quite some time now and have found it to be one of my most used web applications. The ability to access my bookmarks from any browser increases my productivity exponentially.
The other use for del.cio.us that is helpful, is using it as a topical search engine. If i am looking for something in particular, I'll use a tag search and chances are I will find exactly what I am looking for in a few short minutes. The reason this works - is that del.icio.us is essentially a human powered search tool, instead of a text-matching tool, like Google.
Oh - and with regard to my earlier post about Melvil Dewey... well, it seem, librarians are embracing del.iciou.us and other tagging sites.