PCAS
I bought the Zaon MRX a few days ago and used it on a flight yesterday from EGLM-EGBP and return. The unit worked well..it warned me of traffic before Brize LARS (who were well on top of the general traffic situ)..but on departure from Kemble it gave me an ALERT (i.e closer than ADVISORY) which immediately led to mental images of me in a mangled heap of alumunium alloy 800ft below! hence a little panic set in and I dropped my immediate task in hand. As it turns out the traffic was in the overhead join.
So, like lots of technology, they have to be used appropriately and interpreted with sensible judgement; and ultimately they must not replace any degree of situational awareness. As one of the other posters has indicated, they it can definately take over the flight resulting in your lookout becoming nothing more than a hasty reaction to its output.