Jimshutt, you need to drop the "radar" off the title of this thread cause there is no radar in PCAS.
I have Zaon XRX (the dearer one) in the Bonza, but have had nothing to do with MRX (the cheaper one).
The XRX is a good bit of gear for the money, IMHO. It is generally effective in alerting potential conflicting traffic. As has been pointed out above it is a passive system and relies on either SSR to trigger the conflicting traffics transponder or another aircraft equipped with and active TCAS system. Therefore it is not of much use away from SSR coverage.
To use the XRX to greatest effect, I think it ideally should be connected to a moving map display such as Garmin 296 - 696. I also think it works best if connected into the aircrafts audio so that you get an audio alert of conflicting taffic. It would be a pity to have the gear and then hit something cause you didnt happen to look at it at the right time.
In the Bonza, it has a bit of a blind spot to the rear, but that is not a big issue for me as there are not too many aircraft that are going to rapidly overtake me from behind - that aren't equipped with their own TCAS.
In a mostly busy circuit traffic situation, you would have to play with getting the settings right so that it only alerted you to genuine conflict, otherwise it will drive you mad going off all the time and you will either ignore it or turn it off.
So, how good is it? Well .....
1) going into my favourite aerodrome (YCAB) I usually pick up all the bogies before I enter the circuit
2) it makes it easy to find Jaba in the Plastic Parrot when trying to formate on him!
Dr 