Yes, it is useful
Matt, I have the XRX and I always use it when I'm flying. It does seems to give the odd spurious reading and the voice alert system seems to have a habit of cutting in just when I'm talking to ATC. However, I do think it is invaluable and it has picked up a number of close flying aircraft that frankly I was not in danger of hitting, but I was really grateful to know were there.
If you are vigilant and scan with the Mk 1 eyeball obsessively, then the Zaon gives you the odd helpful cue which I think makes you safer. If you rely on the Zaon and don't keep your head outside the aircraft, then it's probably less safe than not having one.