I did a lab exercise (part of physics curriculum) with a PID controller once. It controlled a small electric motor with a small flywheel (i.e. not much inertia). We hand-tuned the parameters with potentiometers. The effect was quite impressive: the flywheel kept spinning at the same speed when we touched it, even with moderate force. It felt like there was a huge motor driving it because the RPM just wouldn't budge. It was still not hard to stop it completely (small motor after all), but it felt very stiff before that point.