The last time I had a discussion with a TDD'er about this (a few days ago), his objection was that mocks don't work with static classes and methods, because you can't instantiate them. And that was really his only objection. I have noticed that mocking can be very intrusive; it requires virtual methods or interfaces everywhere, unless you want to rewrite method calls at the binary level. None of these things are required for functional-style programming.