I'm working on a very agile development system, a small number of people with my doing the vast majority of progaming myself.

I've gotten to the testing phase and find myself writing mostly functional level testing, which I should in theory be leavning for our tester (in practice I don't entirely...trust our tester to detect and identify defects enough to leave him the sole writter of functional tests). In theory what I should be writing is Unit level tests.

However, I'm not sure it's worth the expense. Unit testing takes some time to do, more then functional testing since I have to set up mocks and plugs into smaller units that weren't design to run in issolation. More importantly, I find I refactor and redesign heavily-part of this is due to my inherriting code that needed heavy redesign and is still being cleaned up, but even once I've finished removing parts that need work I'm sure in the act of expanding the code I'll still do a decent amount of refactoring and redesign. It feels as if I will break my unit tests, forcing wasted time to refactor them as well, often due to unit test, by definition, having to be coupled so closely to the code structure.

So.is it worth all the wasted time when functional tests, that will never break when I refactor/redesign, should find most defects? Do unit tests really provide that much extra defect detetection over through functional? and how does one create good unit tests that work with very quick and agile code that is modified rapidly?

ps, I would be fine/happy with links to anything one considers an excellent resource for how to 'do' unit testing in a highly changing enviroment.

edit: to clarify I am doing a bit of very unoffical TDD, I just seem to be writing tests on what would be considered a functional level rather then unit level. I think part of this is becaus I own nearly all of the project I don't feel I need to limit the scope as much; and part of it is that it's daunting to think of trying to go back and retroactively add the unit tests needed to cover enough code that I can feel comfortable testing only a unit without the full functionality and trust that unit still works with the rest of the units.