Mats Helander writes about realizing the value of the "red" in unit testing. I'm afraid I don't fully agree with Mats' analysis of "red" being more valuable than "green". From my point of view, it's the change in color that matters most, not the actual direction of the change. In other words, while the constant green gives you confidence and the occasional red gives you even more confidence, in my opinion, it's really the change of color that is the core of all this.
Dion is right on the money. I've seen way too little unit tests out there and way too much of those have been written with the "I'll have to unit test this class so let's see what public methods it has" mentality. Don't get me wrong; it's better to have decent unit tests than none at all. It's just that if you're going through the trouble writing all those tests, why not make them better while you're at it? It's the behavioral tests that give you the new insights about your design. It's the behavioral tests that give you the most confidence. It's the behavioral tests that matter most when it comes down to return on investment.
The September issue of the JavaRanch Journal features a tutorial of mine, Driving on CruiseControl, which shows you step by step how to set up the open source continuous integration server, CruiseControl.