Not worth running one IMO, bit of an arcaic way of trying to detect knock - and if it actually worked properly why so many melted pistons? It's just a microphone with some amplifier circuitry and a shitload of filtering to try to make some sense of all the crap it picks up. Your ears are a much better tool!
To do it properly there are systems that look at induced voltages across the plugs - that's how the system in my Saab works, allows it to adapt the boost and ignition curves to the type of fuel you're running. Fairly sure that's the standard way of doing things now, either that or measuring cylinder pressures directly with pressure transducers in the HG - though from what I've seen that's more of a development tool, not sure any OEM uses that.