Ideally, the more information you maintain about a visitor or customer, the more ways you can reach and tailor your message for maximum effectiveness.
Personalization can be effective with even a small amount of data like name and email address. Or, your online can community can service their own profile as shown in this example.
Our approach is to gather only data that is necessary in communicating your message. Sometimes the solution can be as simple as using a "cookie" to identify the general characteristics of the visitor.