I’ve been using del.icio.us to tag things for a couple of months now. The immediate benefit is being able to see thing anywhere that you tagged on another computer, like moving between home and work, as opposed to traditional bookmarking which is saved to the computer that it was done on only. You can also see your tags at public computers like the LIBRARY. People can also share tags!
On the flip side, libraries need to add more icons for tagging to their website and catalogs to encourage the tagging of these items by our patrons.