I make a living doing the *other* kind of testing: acceptance tests, system tests, functional tests, whatever you want to call it. (Sometimes erroneously called "QA".)

I don't think that the first purpose of unit tests is to find bugs. I think that the first purpose of unit tests is to provide some assurance to the programmer, the maintainer, and the tester that the code in question is in fact sane. That the code has been exercised in some reasonable and thoughtful fashion before being inflicted on the user (whoever that is).

Put another way: write enough unit tests to prove to yourself and to those who come after you that you really did pay some attention while you were coding. You can't write a test that will catch every bug. Trying to do so is a waste of time.

Put yet another way: as a programmer, your job is the trees; as a tester/QA guy, my job is the forest. We're both walking around in the same woods, but we're paying attention in very different ways.