One thing I have noticed with over zealous testers is to get 100% coverage they mock everything and test implementations. Then you end up writing 2x the code to get anything done. I think it's a net negative. I have more often had to rewrite tests in these environments then have them actually save me from shipping a bug.
This is why I very heavily favour functional tests or broad use-case based integration tests that are implementation agnostic. Save your highly coupled unit/property based tests for algorithmic code or weird edge cases when things are settled. More generally, even many TDD people advocate “spike then stabilise” as an approach.
That is such a sweet feeling - doing what you feel is good test suite (maybe not TDD perfectionistic 100%but good) then few months later you add a new feature and the tests find some subtle bug that would have affected production.
