We initially only had integration tests, because many people think they're better. I get it: itests use the real plumbing, so they're more representative of your runtime. But they're slooooow -- especially the tests that involve the DB (which is most of our itests).

So we started adding unit tests. Utesting code that wasn't written for utests is painful: you often need to choose between refactoring or just patching the hell out of it. The latter is highly undesirable, since it leads to verbose tests, failures when you move a module, and the inability to do blackbox testing.

But utests encourage our new code to be clean and readable. We've found that functional programming is much easier to test than object-oriented, and is easier for engineers to grok. We just sprinkle a little dependency injection and the whole thing works nicely.

Itests have their place, but utests lead to faster feedback and more readable code.