it's useful as long as you appreciate the limitations, and like chip says having the "rich" light on won't mean it's neccesarily rich enough. But you can tell if something is badly amiss as I said yesterday - MAP sense pipe coming off the fuel pressure regulator for example will show lean on boost no problem
I built my display myself, with scale modifications to suit what I want so I know how the LED characteristic relates to the sensor, which does make it a bit better, but on a commercial unit you've got little way of knowing much more than "lean" or "rich"