One of the "Aha!" experiences/epiphanies of a programmer I was working with, was the realization that a unit test only needs to test a "unit". He had been setting up real-life data, when a little "fake" data would verify the desired behavior.
Unit testing also makes the code more robust, because side-effects that might not be seen in a system test, can be found by unit tests exercising all the edge-cases for each function or behavior being tested—particularly testing situations that are hard to replicate in the real system.
