Do you find that you are able to reuse your mixins easily, or do they evolve slowly to become general and reusable components?
We came up with this when we saw that several of our components did too much and we had to much copy&paste going on, so we haven't actually created many from the ground up. But the one we created using TDD from the ground up has proven to be very reusable. So I guess my answer is both, but in the beginning I believe it's primarily the latter.