In addition to sounding better than “MyWeb2.0 Luky Charms”, del.icio.us allows you to export your bookmarks easily. MyWeb2.0 does not.
I’d rather write off all the bookmarks i’ve created in the meantime than be tied down to a single service.
MyWeb2.0 does a few snazzy things that I liked. Saving a copy of your bookmarked pages in the cache was nice. The ability to publish links to groups of users was nice. But ultimately, it was easy to export from del.icio.us to test my own dogfood, but the realization that I could never go the other way led me to cut my losses and switch back.