It's one of my most used web services too, for personal bookmarking. There is no more disappointing feeling in the world then looking back for an article I read a few weeks ago and realizing I forgot to save it in del.icio.us.
When I think of the huge graph of information they must have about so many pages, I just believe the haven't managed to do anything better on the macro level. How much of the (useful) web must by now be tagged in del.icio.us? Why has not the service become the de facto place to find information based on tag or topic?
The only silver lining I see is that a) the data exists, so such an application could evolve in the future, and b) at least Yahoo didn't muck it up and somehow ruin what continues to be a useful service on the personal level.