I can give you my experience with TDD: You generally don't need to change directions very much. TDD will expose bad interfaces or interfaces that aren't abstr

I can give you my experience with TDD: You generally don't need to change directions very much. TDD will expose bad interfaces or interfaces that aren't abstract enough early on because you can't test them easily. And if you do decide to change directions you sit down and write the tests and then make the changes until your code works and the tests pass. Generally you want to write tests before code and make your code pass the tests. How many tests and how much coverage is up to you. It's important that the tests are small and brittle so you can pick out as many breakages as early as possible. You use the API in your tests. When the tests start to fail you know your API has changed. It gets a bit more difficult with web APIs if they don't provide a test environment.