Donald Norman posted an article called Ad-Hoc Personas & Empathetic Focus that includes some great examples of personas in action. Norman suggests that made up personas that ring true are better than none, even though Forrester Research disagrees with this approach.
I particularly liked this explanation of “Empathetic Focus” in site design:
“The purpose of the Persona, I believe, is to add empathetic focus to the design. Empathetic focus. By focus I mean that the design must be clean and coherent. It is not a collection of features added willy-nilly through the life-span of the product, even if each feature by itself makes sense. Rather it is having a clear image of what the product is meant to be — and what it is not meant to be — and rejecting features that do not fit, only accepting ones that do. By empathy, I mean an understanding of and identification with the user population, the better to ensure that they will be able to take advantage of the product, to use it readily and easily — not with frustration but with pleasure.”