The best people to ask about your product are the customers themselves. That is, after all, the primary thinking behind focus groups. How can a customer community improve on that? By allowing you to see how customers talk about the product to each other, without the pressure of a focus group situation. You have a longer time frame to look out for product problems and improve user experience. Are the problems that a user has with your product the same in the 3rd month of ownership as they were in the first week?

Your customer community is also a great place to find inspiration for your marketing. You might think you know what the best features of your product are, but a few months of watching how your customers talk about the way they use your products might open your eyes. When you have thousands of customers talking about your products, they're going to come up with angles that even the best marketers could never think of on their own.