I had heard of del.icio.us before. It's hard not to notice all the little buttons found on every site you go to! Again, I found it an interesting site, but it really depends on how you use the web.
My first thought was that this was the perfect type of site for a (I just heard this word earlier today and loved it) "cyberloafer". Someone who spends a lot of time in front of the computer while at work and tends to wander around the web randomly when they should probably be doing something productive. It makes all of your bookmarks portable so you can find all your favorite sites without memorizing the url or bookmarking them on a work computer.
I thought about it a bit more and realize it could also be used for work things. It would be great to create an account for our branch and use it to bookmark library related sites - Readers Advisory pages, official author pages, upcoming movies based on books websites, etc..
I do feel the site is a bit overwhelming. It is something I would have to use for awhile before I became comfortable with it. And it really depends on what you use the web for. Results on delicious can be overwhelming if you don't know how to search on it.