I tried Google's speech application for finding business phone numbers. I successfully found a business in Boulder, CO, in my first attempt. I'd need to work with it more before I passed judgment on how functional it is. What I found remarkable is the presentation. Here's the initial greeting.
"Calls recorded for quality. GOOG 411 experimental. What city and state?"
It's quick and businesslike. No effort to be cute or fancy. Let me be the first to say (at least I'm think I'm the first) that Google has apparently tried to capture the visual presentation of its web page in an auditory presentation. It succeeds. Its web page is just a white page with a logo, an input box, two buttons, and a small number of links. If you were trying to translate that visual presentation into a VUI you couldn't do a better job than Google has.
By being simple and almost terse, Google created a unique, differentiated experience. If its speech browser's performance is as good as its web site, it will have a winner.