It's not about telling you when you're done a feature. It's about leaving step 17 with a set of tests so that the next person in the code can tell when he's done without doing steps 1-17. And you'd think it slows you down, but it really doesn't. Some advantages of TDD are less context switching (you can test your code without even leaving the code itself) and a high degree of focus (every atomic subtask has a very clear completion criterion: fix the failing test). Those are things solid developers love.