Jason at 37signals has an interesting post about why they don't use personas:
Every product we build is a product we build for ourselves to solve our own problems. We recognize our problems aren’t unique. In fact, our problems are probably a lot like your problems. So we bundle up the solutions to our problems in the form of web-based software and offer them for sale.
We recognize not everyone shares our problems, our point of view, or our opinions, but that verdict’s the same if you use personas. Making decisions based on real opinions trumps making decisions based on imaginary opinions.
I’ve never been a big believer in personas. They’re artificial, abstract, and fictitious. I don’t think you can build a great product for a person that doesn’t exist. And I definitely don’t think you can build a great product based on a composite sketch of 10 different people all rolled into one (or two or three).
In addition to the post itself, the comments are really well-written with good points made in both directions.
For me, what's interesting is that I am known for not being a big fan of personas, and yet I think Jason is completely off the mark in his criticism. Or, to be fair, he highlights good reasons why 37signals doesn't need to use personas, but doesn't seem to recognize that those reasons don't apply to many other teams - 37signals is not a representative company. They are the exception, not the rule, because they strongly believe that they are the users of their own products. Not only is this unusual, in my opinion it's also not sustainable.
The other mistake he makes is that he thinks personas are meant to replace talking to real people. Of course, the truth is the opposite - personas are the output of talking to real people. For most organizations, it's just not feasible for every person in the organization to talk to real people and have the skills to successfully interpret what they actually mean and not just what they actually say. For the people who do talk to people and do have those skills, we need a way of communicating the results of those discussions to the rest of the team. Personas are one way to do that.
(Of course, it goes without saying that like any technique, there are teams that do personas poorly, but that is a reflection on that team, not the technique)