I only started using del.icio.us a few months ago, but I'm already finding it to be an indispensable service. I've long used Google Bookmarks as I have more than one computer so I like to be able to access the same set of bookmarks from all of them, and this serves me very well for this purpose. I don't have any plans to abandon it entirely (at least not for the foreseeable future).
But del.icio.us, at least for me, serves a slightly different purpose. Google Bookmarks is what I use to bookmark sites I will want to return to in the near future, and may visit regularly. By comparison, del.icio.us is what I use to bookmark sites I am unlikely to use regularly, but may want to visit under certain circumstances - things like handy tutorials or bits of code I can see myself wanting to use in future. It's great for keeping an archive of things like that.
By comparison, Google Bookmarks is great for keeping track of the websites I'm likely to visit fairly often (and there is a LOT of them) where I wouldn't want to dig through the huge numbers of sites I've bookmarked on del.icio.us.
Also, del.icio.us is a handy search tool. You can search through all the bookmarked sites in the same manner as any other search engine. This is handy because everything in there is something that has been submitted because they may want to refer back to it in future, so you're less likely to come across something that isn't useful or relevant. The tagging system means your search relies on other people's tags, which should mostly be relevant.
So, give it a try. There's a Firefox extension or Flock has built-in support. And many other browsers no doubt have the facilities to support it. It's one of those tools that makes browsing easier and more fun.