Personas solve the age-old problem of the Internet - designing for everyone. Your audience is not all the same, and you won't be able to design for them unless you understand how they differ. Equally you can't design for each and every audience member either - and that's where personas come in.
Personas are basically a model of your customers - actors that you can use to 'walk through' the design and ensure it makes sense, before you reach a visual stage. They are particularly useful at the very early stages of design, to ensure you are setting the right design direction.
We do believe that personas can be very useful in addressing key design questions and helping you move forward; but we don't believe they are crucial, for many projects. It really depends on how much time you have, what you need to achieve and what you know already.
Are personas right for your project? Call us - we're happy to discuss that with you and give you some free guidance.