I love posts like this one by my friend John Socha-Leialoha. An excerpt will show you what I mean:
Yesterday and today I did two major refactorings that before would have required very careful work and testing. However this time it was nearly painless. The unit tests I’ve written now do that work for me. So when I did the refactoring yesterday, roughly 10% of the unit tests failed. It only took me about 15 minutes to fix the code so all the tests passed.
Gotta love that!