[jaredsohn]

Good tests reduce your cognitive load; you can have more confidence that code will work and spend less time worrying that someone will break it.

BTW, I think above are the best metrics to use for tests. Actually measuring it can be hard, but I think keeping track of when functionality doesn't work and people break your code is a good start.

And I think all of this should be measured in terms of doing the right thing business logic-wise and weighing importance of what needs testing based on the business value of when things don't work.