The XRX is subject to a lot of potential errors, and it's very easy to get wrapped up in gimmicks in the cockpit that distract from the more important task of flying the airplane (and looking for traffic). That said, I've had at least four recent near mid-air's of which I was alerted using a Zaon PCAS. Each occurred at night in areas with poor control and an unusual method of control, and in each case I would have had no way of knowing about the traffic, save for that unit.
I've also had hundreds of false warnings, "ghost" images, etc. Given the circumstances, I'm very certain the near midairs (which were same altitude, less than a tenth of a mile) were genuine warnings. In two of those encounters I saw the other aircraft in the dark...one as it's wing passed under mine.
My assessment is that it's better than nothing. I'm more familiar with TCAS equipment, and the PCAS doesn't come remotely close to the quality and capability of TCAS...but it doesn't come close to the cost, and is a whole lot easier to install, too. Realistically, few (if any) are going to install TCAS in their light single engine airplane...but PCAS offer an option.
An important difference with PCAS is that it's not active; it's passive. Whereas TCAS interrogates other aircraft's transponders, PCAS does not. If you're not in an area where other aircraft are being interrogated by either ATC radar or TCAS units, then you won't see them on PCAS.