Kathy Sierra explains the conundrum of Buying and Using criteria in The real secret to success: "The secret is simply this: you have a much better chance for success when your business model makes what's good for the users match what's good for the business, and vice-versa."

There are certain features in your product that must be present or no one will buy it; these features are buying criteria. Features like a camera in your phone. Sales-oriented companies tend to be really good at loading up their products with buying features. But what about the user? If the end-user customer doesn't use it--or doesn't like using it--you won't get renewals and referrals. Engineering-led companies tend to focus on using criteria to the detriment of buying criteria. As Kathy points out, we need to find the right intersection between both to get the product both bought and used.

Joel Spolsky pointed this out in a different context. Buyers requested the option of custom fields so they could tailor the product to their existing environment. He writes, Historically, I am opposed to custom fields in principle, because they get abused. People add so many fields to their bug databases to capture everything they think might be important that entering a bug is like applying to Harvard. End result: people don't enter bugs, which is much, much worse than not capturing all that information.

Joel knows that offering a bunch of custom fields will make the product more difficult to use and he’ll lose renewals and referrals. The solution is to recognize the source of the request: is the idea coming from buyers or from users? Will people buy without it? If not, can we implement it 'skinny' so that it won't mess up users?

If we listen only to the buyers, we end up with a Zune instead of an iPod. But if we listen only to the users, we'll never get the thing sold in the first place.

Which features in your product are buying criteria? Which are using criteria? It's the intersection between the two that results in a product that people love.