I started test driven development with my students today, who are learning unit tests really early in their programming careers (3 weeks in to Javascript). It's really good, it makes them slow down, run their code often, work incrementally, and get their expectations out of their brains and in to the code. My idea is that I wish I learned TDD from the beginning, so I'm trying to do for them what I wish had been done for me. I love them.