Apart from the proper dictionary definition, Evanescence is a rock band with great lyrics. You don't have to be a depressed or be into rock to appreciate their music. Even if you're a content and happy person, you can to some extent relate to their songs. If they weren't any good, they wouldn't have so much publicity, think about it, how many other bands in the same genre have succeeded like them.
Eg) My Immortal - lost love?
Going Under - finally letting go?
Bring me to Life - self recognition?
Breathe No More -
Self-improvement is for the weak, the strong seek self-destruction.