I was an early del.icio.us user and initially thought it was pretty cool but after a while stopped using it. Google has made most hand indexing of web content irrelevant because you can generally find things again if you vaguely remember where you saw them the first time.

In a way it's analogous to the schema on write vs. schema on read approaches to data analysis. If you don't know the end use of data, schema on read is usually a lot more efficient.

Edit: left out 'user'