Should we use personas in user stories

Question

The book User Stories Applied contains single page discussing Personas. The definition of persona from the book is:

A persona is imaginary representation of a user role.

It futher discuss definition of the persona:

Creating personas requires more than just adding a name to a user role. A persona should be described sufficiently that everyone on the team feels like they know the persona.

It also recommends to find a photo on Internet or in magazine and use this photo for persona so that everybody can clearly imagine persona working with the application.

Ok. All these ideas sound good. It can be fun to define personas to user roles but is it worth it? Is there any real or measurable quality or increased efficiency when using them?

Do you have any good examples where personas really help the development team? Do you use personas in user stories?

Edit:

I have found nice article about personas in MSDN.