I think the general idea is that you should be picking your input data to be as evil as possible, so that you're explicitly testing the edge cases; if you use r

I think the general idea is that you should be picking your input data to be as evil as possible, so that you're explicitly testing the edge cases; if you use random data you're not thinking about this aspect. Fuzz testing is a separate thing, and should be done too. But it's (in my experience) supplemental to well-chosen hand-crafted input. It's hard to do TDD or ensure you have all your edge-cases covered if you're using randomized inputs. For example, for datetimes, you can pick something nasty and unlikely like 1999-12-31T23:00 Pacific Time, which will read as a different day, month, and year if you have your timezone logic broken somewhere (e.g. you're parsing it as a naive UTC datetime somewhere). If you test randomly, that "dates at the end of the year parse incorrectly for 8 hours" bug is unlikely to be found.