Tim and I are no strangers to controversy and debate. As agile coaches, we get challenged all the time. "TDD is a waste of time, it doesn't catch any defects, you don't want to go past 70% coverage," and so on. They sound like excuses to us. But we're patient, and perfectly willing to have rational discussions about concerns with TDD. And we're the first to admit that TDD is not a panacea.
Still, we haven't yet seen a better developer technique than TDD for shaping a quality design and sustaining constant, reasonable-cost change over the lifetime of a product. Yet: If you show us a better way, we'll start shouting it across the rooftops. We're not married to TDD, it just happens to be what we find the most effective and enjoyable way to code.
Many of the challenges to TDD come from the crowd that says, "write the code, then come back and write some unit tests." This is known as POUT (Plain Ol' Unit Testing), or what I prefer to call TAD (Test-After Development). The TAD proponents contend that its practical limit of about 70% coverage is good enough, and that there's little reason to write tests first a la TDD.
70% coverage is good enough for what? If you view unit testing as a means of identifying defects, perhaps it is good enough. After all, the other tests you have (acceptance and other integration-oriented tests) should help catch problems in the other 30%. But if you view tests instead as "things that enable your code to stay clean," i.e. as things that give you the confidence to refactor, then you realize that almost a third of your system isn't covered. That third of your system will become rigid and thus degrade fare more rapidly in quality over time. We've also found that it's often the more complex (and thus fragile) third of your system!
And why only 70%? On the surface, there should be little difference. Why would writing tests after generate any different result from writing tests first? "Untestable design" is one aspect of the answer, and "human nature" represents the bulk of the second part of the answer. Check out the card (front and back).