The tests has no value by themselves: My most successful project didn't have any tests
In DevTeach, we had a panel that Kathleen Dollard has covered in depth, in which we talked about what the bare minimum aspects of Agile project would be. The first thing that was thrown up was testing.
That is quite predictable, and I objected to that. One of the things that bother me about much of the discussions in the agile space is the hard focus of tests. Often to the exclusion of much else.
My most successful (commercial) project was done without tests, and it is a huge success (ongoing now, by the way). A previous project had tests, quite a few of them, and I consider it a big failure (over time, over budget, overly complex code base, a lot of the logic in the UI, etc).
Update: I wanted to make clear the distinction between Agile and TDD. I consider the project without tests to be a fully agile project. The project with tests was a heavy waterfall project.
I think that I can safely say that it would be hard to accuse me of not getting testing, or not getting TDD. Not that I don't expect the accusations anyway, but I am going to try to preempt them.
I want to make it explicit, and understood. What I am riling against isn't testing. I think that they are very valuable, but I think that some people are focusing on that too much. For myself, I have a single metric for creating successful software:
Ship it, often.
There are many reasons for that, from the political ones and monetary ones to feedback, scope and tracer bullets.
Tests are a great tool to aid you in shipping often, but they aren't the only one. Composite architectures and JFHCI are two other ways that allow us to create stable software platform that we can develop on.
Tests are a tool, and its usage should be evaluated against the usual metrics before applying it in a project. There are many reasons not to use tests, but most of them boil down to: "They add friction to the process".
Testing UI, for example, is a common place where it is just not worth the time and effort. Another scenario is a team that is not familiar with testing, introducing testing at this point would hinder my topmost priority, shipping.
Code quality, flexibility and the ability to change are other things that are often attributed to tests. They certainly help, but they are by no mean the only (or even the best) way to approach that.
And finally, just to give an example, Rhino Igloo was developed without tests, using F5 testing style. I applied the same metric, trying to test what it does would have been painful, therefor, I made the decision not to write tests for that. I don't really like that code base, but that is because it is strongly tied to the Web Forms platform, not because it is hard to change or extend, it isn't.
Okay, I am done, feel free to flame.