The New York Times publised an article about how Persona based design research played a role in developing and designing the cars for Ford's 2010 sales season: http://www.nytimes.com/2009/07/19/automobiles/19design.html

Here are a few memorable quotes from the article: 

We found in the past that if they didn’t understand the buyer, designers would just go off and design something for themselves

You get a common focus for everyone from the clay modeler to the chief executive

There are a few details I'm not comfortable with in the article. It's the use of the words imaginary and imagination for building the user profiles. The persons on which the persona's/profiles were built on certainly do exist even if many personality traits and user needs from different persons were combined into one or two persons. It is similar to a movie based on the true story where the identity of the characters were changed but the characters themselves were not 'imagined' or 'imaginary.'

Save for the above indescripencies concerning the words imaginary and imagination, the article overall highlights the needs for good user research and stratetgy in the product design and development cycle.