The worrying thing about writing tests is that there are numerous accounts of people introducing tests into their development process and.. ending up with even more junk code to support than they had to begin with! Why? I guess, mainly because they lacked the knowledge and understanding needed to write good tests: tests, that is, that will be an asset, not a burden.
Reference: Bad Tests, Good Tests, Tomek Kaczanowski, http://practicalunittesting.com/btgt.php