I've never used one for anything. I think part of the appeal was that it worked something like Digg or Reddit - the social aspect.

Without the appeal of seeing what other people have bookmarked, it would work just as well to maintain my own list on an HTTP server. Then I can access it from anywhere just the same.

Before native bookmark sync, it was my way of ensuring I knew how to get to all the same places on multiple computers.

Also, the 'suggested tags' feature worked really well most of the time, which was awesome, because I otherwise would have never taken the time to tag my bookmarks.