(I'm assuming that you're using "functional test" to mean a test involving the whole system or application being up and running.)
I would unit test new functionality as I wrote it, for three reasons:
- It helps me get to working code quicker. The turnaround time for "unit test failed, fix code, unit test passed" is generally a lot shorter than "functional test failed, fix code, functional test passed".
- It helps me to design my code in a cleaner way
- It helps me understand my code and what it's meant to be doing when I come to maintain it. If I make a change, it will give me more confidence that I haven't broken anything.
(This includes bug fixes, as suggested by Epaga.)
I would strongly recommend Michael Feathers' "Working Effectively with Legacy Code" to give you tips on how to start unit testing a codebase which wasn't designed for it.
