Just to be clear: Bill's opinion on this isn't universally shared at JTV.

When you're two guys in a garage, unit tests probably slow you down. But at some point, they begin to dramatically speed things up -- working on someone other person's code becomes orders of magnitude easier when there's a comprehensive test suite to validate your changes.

The problem is that at some point between first employee and a dozen (or more) engineers, you start to wish you'd spent a bit more time on testing code. Writing those tests from scratch becomes "a project", and it only gets done with herculean effort.