Well it started as a better UI than Google's (I said too much here), so I started designing the UI with search as a focus. Than later it evolved.

As for the usable thing, most of UIs I design are usable from the start, in the sense that they don't suffer the designer syndrome.

If you want to see a scifi that suffers usability

Mozilla's Aurora Concept http://www.adaptivepath.com/aurora/