I think Del.icio.us has the potential to be a great tool if used properly. If using it for work, or teaching I think it is really important to keep your personal sites separate. Also, if you are using it for teaching purposes, getting exam questions or homework problems it might be a bit of challenge since it is a public site. Students as well as other professionals have access to the web, and your created site. A student could easily complete a google search and has the possibility to running in to your potential exam questions.
Del.icio.us is a great research tool. A user has the ability to organize valuable sites for their current research topic, past and feature projects. Also, a user has the ability to search for other users completing similiar research and the ability to review the sites that they find valuable. Del.icio.us has the potential to become a research social networking site.
Right now, I'm currently using Del.icio.us as a bookmarking site. I have started using to organize my daily website visits.