I've been using del.icio.us for the last three years as my primary search tool. Google/Yahool are great for the generic stuff but for the meaningful stuff I use del.icio.us. Why? Easy, most of the bookmarks to web pages are intelligently filtered, noted, and tagged.
Hey, would you put crappy music on your ipod?
Social Bookmarking is one of the easiest tools for users to adopt. It only takes a few people to build up an incredibly rich library.