I just want to say, it's always a nice feeling when I get all-green output from rspec and jenkins. The problem is that tests, like your code, are subject to the laws of entropy that comes from bit-rot.

So, I test things that matter and don't change too often - core business logic.

100% test coverage is just a goal, a bar to aim for.

And I'm totally with Zed Shaw when it comes to TDD - not worth it when you're still trying to get a full understanding of your problem domain.