Testing it is indeed more difficult. It requires high restrictions on the light condition and the surface type, cause its using the colour sensor for the proximity of the surface for tilting the motor. (Have to balance it at the beginning).
Anyway, its fun to see it working. There's a good article on the PID controller concept, which applied in this project. (http://www.inpharmix.com/jps/PID_Controller_For_Lego_Mindstorms_Robots.html)
