Here's my response:
I've worked in plenty of teams where the coders have written unit tests. Typically at organisations which explicitly care about coding. Less so at banks, service or entertainment organisations. On a reasonable proportion, and most especially on agile teams, the coders have written very large numbers of unit tests that act as a scaffold to the code (that is, very large as compared to the expectations of teams that don't use unit tests, so perhaps this is tautological) . More than a simple framework, the tests also act as a way to frame thinking about the code, to consider it before it is made, and to experiment with it when it is under construction.
Such teams are (in my experience) universally proud of their unit tests, and actively show them off. Their code tends to be better, too. When I've worked with teams who are shy of showing me their unit tests, and shy of letting me review them, then (in my experience) the code is universally duff. I'd rather work in a team that has no unit tests than a team that says it does, but won't show me (as a project member who is interested in testing) the tests.
I have worked with teams that write unit tests, but don't run them. This sounds bizarre, but tends to be a problem that creeps up on teams. I've seen it as a result of commenting out unit tests that break because of a non-code-related change (without replacing the test). Be aware also of unit tests that don't test anything important, or which pass with or without the code in place. I have heard of (but not worked on) teams where the testers wrote all the unit tests, and the coders wrote the code. I guess I'd hope that the two groups work closely enough that tests and code could be written together.
It's all to easy to let good unit testing and the resultant relatively-clean code lull one into a false sense of security about the viability of the system as a whole. If I work with a team that is using lots of unit tests, I (very broadly) take this approach:
- reviewing the existing unit tests
- being sure they're run regularly
- trying not to duplicate them too much with my own / the customer's / other team's confirmatory scripted tests
- using exploratory / experimental / diagnostic approaches to pick up and dig into all those unexpected risks and surprises
- working closely with the coders to enhance, streamline, and otherwise improve their unit tests