As it happens, I read a paper last night on this very subject. The authors compare projects within four groups at Microsoft and IBM, contrasting, in hindsight, projects which used both unit testing and functional testing and projects which used functional testing alone. To quote the authors:
"The results of the case studies
indicate that the preview release
defect density of the four products
decreased between 40% and 90% relative
to similar projects that did not use
the TDD practice. Subjectively, the
teams experienced a 15 to 35% increase
in initial development time after
adopting TDD."
This indicates that it is certainly worth doing unit testing when you add new functionality to your project.
