Jared Spool on personas
Over on the UIE blog, Jared Spool asks, "When Should You Use Personas?" He makes a good point that personas are essential when you're working as part of a team. If it's just you in a dark room building a web site, you're unlikely to argue with yourself about who you're serving and how you should go about it. Personas help build consensus when people with varying opinions come together to work.
Make sure to read Dave (Heller) Malouf's comment as well. Smaht man.