I use Test-Driven Development (TDD) to produce most of my production code, so I write my unit tests first unless I'm working on untestable legacy code (when the bar to write the tests is too high).
Otherwise, I write functional-level acceptance tests that the code shall satisfice.
Writing the unit tests first allow me to know exactly "where I am" : I know that what has been coded so far is operational and can be integrated or sent to the test team ; it's not bug free but it works.
