I think one of the main benefits of writing tests before implementation rather than after, is that it forces you to think about making a testable design. But if you are already in the habit of making testable designs, then IMO TDD isn't worth it, because front-loaded tests can slow down your iteration speed, and become a drag if you are trying out a couple different designs before settling on one. Once you know how to write testable code, you're just as well off writing the tests afterward - works fine for me at least.