Those cheap gauges that run off the 3-wire lambdas are essentially a voltmeter, 0.1-0.2v=lean, 0.8-0.9v=rich, 0.3-0.7v=stoich, or thereabout. So they are pretty usefull, you just don't know exactly what the ratio is, but you can expirement to sort of work it out.
I've used mine to get it to run...