I have had a delicious account for a while, and I think it is a great tool. It is extremely convenient to be able to access my bookmarks from any computer (or any other tool one can use to access the Internet), instead of only having them on my home computer. I also like that I can categorize them and quickly browse through the pages I have marked on a certain subject. The cool thing about organizing with tags instead of folders is that I assign a variety of tags to a site, instead of having to choose just one folder in which to store it. Delicious helps me also to keep track of cool pages that I find which I may not want in my list of frequently used bookmarks, but that I would want to remember and come back to at a later time.
I have not really explored much with the social networking aspects of delicious, however. To be completely honest, too much social networking goes across the line from being convenient to being another time-waster, in my opinion. I like the way HFAR is using del.icio.us, and I think this is a great example of how libraries can use this tool. We make booklists and pathfinders to distribute to customers; delicious essentially allows us to create multiple pathfinders of online resources which customers can access in one place. How cool!
I do have a gripe, though. Why do we not have a delicious button on our toolbar???? We get to see how great this tool is, but we cannot use it at work, because we have to continually re-install the button to save sites. Personally, I think this is ridiculous and I hope it gets changed!