[xmatos]

Build tests against your app`s public interface. On a web app, that would be your controllers or API.

That will give you good coverage, while avoiding too simple to be useful unit tests.

It's really hard to foresee all possible input variations and business logic validations, but that doesn't mean your test suite is useless.

It just means it will grow everytime you find a new bug and you are guaranteed that one won't happen again...

[bluGill]

On a small app that is a good idea. However on a massive app you need to break things down more. I work on a project with > 10 million lines of code, there is no way any human can understand everything in detail. As a result I need to test at a smaller scale which is not my public interfaces, but my interfaces to other people.