Scott Bellware writes about Microsoft missing an Agilist Persona (in addition to Mort, Elvis and Einstein. I pretty much agree with Scott's views - MS's lack of understanding of the Agile crowd is evident in "fiascos" like the TDD article  on MSDN or even MSF Agile, which is  relatively a light process but still very far from processes such as  XP, SCRUM and the like.

Personas is a very interesting concept, usable as a communication aide, for development teams - as a means to help maintain user focus.

The CHAOS Chronicles 3.0 by Standish group* (www.standishgroup.com)  cite "User involvement" as second most important success factor for development project success  (after executive management support). Indeed many agile methodologies also encourage high customer involvement in development process

During my professional career, I had the chance to work for both product companies and solution companies.  - Why is that relevant you ask? Well, when you work for a solution company you usually have tangible, real-life, customers to work with. You can walk them through early usability prototypes, you can consult with then on problematic requirements, you can have them on site for instant feedback etc. etc.

Things are more problematic  when you work on "shrink wrapped" products - now your customers are much more abstract , and elusive, yes you can still hold focus groups etc. but your you can't have that day-to-day interaction with end-users and customers - enter Personas.

I first heard about Personas several years ago, when I read Alan Cooper's "The Inmates are running the Asylum". The book demonstrates some bad  designs for software-based products and then introduces an approach (Personas...) to help avoid these problems (He elaborates more on the approach in "About Face 2.0").

Personas are basically a way to define archetypes of users. Unlike Use Case Actors which represent a role in the system, Persons try to high-light the characteristics of real users. The idea is to come-up with representative model of a user and to give it a full-bio and characteristics  so as to help the development team understand motivations and relate to actual users. The model for absent users is the reason this technique is very  important for product companies. If you don't have real users come up with abstract ones representing them. Alan Cooper introduced Personas as a means to help designers in the initial phases of product design. Microsoft extended the use of Personas as a communication aide to the full range of the development team (designers, developers, testers, managers, marketers etc.) - which brings more benefits. I highly recommend reading the very  interesting paper "Personas: Practice and Theory" by John Pruitt and Jonathan Grudin which  relates Microsoft's experience on the subject.

While Personas are very important for product companies, it can also be important for solutions development especially on larger projects where only few members of the development team get a chance to interact with customers and user.

When you cannot have the customer on site (or the customer representative doesn't give you the full picture of all user types of the system) you can use Persona-Scenarios as a way to augment user stories (or in other circumstances as an alternative to actor-use cases)

*The Standish Group has been collecting metrics from thousands of projects since 1994. They analyze success and failure factors and publish them on yearly basis. For example, while our ratios as an industry are getting better over the years - as 2004 only about third of the projects achieved their goals both  on budget and on time...