I love them both! My delicious account and I have been through ups and downs. I had it and them I forgot about and then I remebered it and then I lost it. And I recently created a new one at the beginning of this year. My diigo account was very easy to learn. I thought the directions were clear and I followed them without problems. I found that having both open at the same time really slowed down my internet. (Not sure what happenend there)
I think libraries could use these to connect patrons to information. I also think this is a great way to see what your patrons are interested in (keeping privacy laws in mind of course). I have see bookmarking sites used for classes. I attended a webinar that showed looks of resources for educators. At the end of the webinar the instructor gave us the delicious link for all the sites. It was nice to have an online copy that would be maintained by someone else.
One major drawback to bookmarking: The information is only as good as its internet life. If someone's page goes down or gets hacked you've lost all that information. If you had a hard copy or created a personal file you would be able to control how long the information is available. But that's the Internet.