NUnit kicks ass!
Compare these two scenarios
Problem: You make some minor modifications which cause major breaks in the code. Repeat the debugging process yet again.
In the next revision of your software you make some minor changes which cause major breaks in the code. Before you even think that the code you changed works, the tests show you first hand that they don’t (you can even debug those features from within the test). Because your tests break you know exactly where the problem starts occuring and can more easily find the problem.
In other news, I alerted AVG to a virus that they didn’t yet detect. Less than 24 hours later they have e-mailed me back telling me that it was I-worm/Palyh.A and that it will be detected starting with the next update.