"Non proper" TCAS with a MDF works just fine in the circuit because it does not switch itself off unless you tell it to. Its a great aid combined with the mark 1 eyeball for picking up circuit traffic. In fact it also works fine for traffic with only mode A because whilst it does not give you a height the diamond is enough to give you a heads up where the traffic is and where it is going relative to you. The automatic alert "traffic, traffic, traffic" gets your attention if you have missed something for whatever reason. It is not perfect, but it is a great help.
I had an occasion recently where traffic appeared on the screen almost exactly paralelling my track and at almost the same speed. It was all too easy to imagine it was a false return. We looked for the traffic in not the best of vis. - neither of us could see it. Our tracks were very slowly converging. Sure enough, it was there, but we could not believe how late we spotted the traffic. A bit of vigorous wing waggling was not enough to grab the other pilots attention.
I recall another occasion down mig alley. The traffic was not working any of the service providers but unervingly almost exactly paralleled our course just holding off on the starboard side. We knew it wasnt working air traffic because in the end we got the jitters because we could not see it and asked if they were receiving a return and working with him. They confirmed they had a contact on radar although occasionally intermittent but were not working the traffic. We never did see that traffic.
Many years ago I was in the hold in IMC - popped out for a brief moment and there was a Warrior coming straight at me. That gave me one hell of a fright. I got a verbal apology from the controller back on the ground who was working us both and who was aware the Warrior had passed straight through the hold at same level as me. I wish I had had TCAS or PCAS then.
The trouble with PCAS, and even TCAS to a degree, is there could be a temptation not to believe it, which is why I have related these experiences. Need less to say believe it every time, even if it subsequently proves to have been wrong. I have only had a few readings that were almost certainly false whilst using PCAS.
I know most of us are not lucky enough to have TCAS but it is well worth the cost and failing that PCAS is a few hundreds of pounds and still pretty good.
There are those that might say it encourages you to keep your head in the cockpit. They would be wrong. If you wish ignore the PFD, ignore the PCAS display, maintain your visual scan - there is no need to look at either. While your eyes are busy outside or with the task in hand the audible alarm will instantly grab your attention, a quick glance at the screen and you know where you should be looking in that moment.