Del.icio.us is an interesting site. I spent just a few minutes trying out the 23 things account before I went on to set up my own account. Then the fun REALLY started. I liked the idea of being able to share the bookmarks on my work computer with the ones I've saved personally at home. (It sounded much easier than trying to remember either the URL or how I found a specific site). So, I piled in a whole bunch of sites into my newly created Del.icio.us account and tried to remember some other sites that I used semi-regularly, but haven't bookmarked. Then I started following links out to other people's accounts. Wow, more cool / useful / interesting sites that I never knew existed! This site is a great way to find hidden gems of the internet without using a search engine. And, there's a small social observer side to me that had fun creating a picture of other users by what they had tagged -- student who travels a lot, avid reader, another librarian? -- I'll never know if I'm correct, but a part of my brain had a great time making up backstories for each of those folks.
While I was having fun, I did note a few downsides based entirely on my own quirks. First, I have a habit of putting commas into a list -- this confused my tags. I'd have a set of "travel" and another set of "travel,"; it was easy to fix, but I had to concentrate after I figured it out to stop myself from continuing the comma problem. Second, I also don't follow capitalization rules when making lists only for myself - so I know I cut down my possibilities by labeling things like London as "london". Again, this is my problem, and not that of the site, but it was slightly annoying to have capitalized and non-capitalized letters separated.
I am sure that this site will continue to be useful to me personally. I am less sure how much I will use it as a professional resource. I certainly can use it for reference work, but I had my bookmarks on my work computer well organized and could have continued with that system without much fuss. It could be very useful in finding new links and sources of information on specific topics, but, as a librarian in a small public library, I don't spend much time compiling topical resource lists for patrons or classes as suggested in the introductory video. I will certainly mention this tool to patrons that I know rely on several computers or mostly public access as a way for them to compile a personal bookmark list that will "follow" them from place to place.