[MillionOClock]

It is definitely not foolproof but IMHO, to some extent, it is easier to describe what you expect to see than to implement it so I don't find it unreasonable to think it might provide some advantages in terms of correctness.

[stingraycharles]

That definitely depends upon the situation. More often than not, properly testing a component takes me more time than writing it.

[radlad]

In my experience, this tends to be more related to instrumentation / architecture than a lack of ability to describe correct results. TDD is often suggested as a solution.