I am surprised given past discussion on this issue some of the misconception here.
Zaon Pcas units are the ones we talk about. There are two versions. The difference is the more expensive one indicates the compass quarter in which to look.
Having used both units alongside Skyforce CAS I can report that their accuracy is outstanding.
Both units give a range and altitude difference. This means it is relatively easy to determine if traffic is getting closer and is at the same height. There maybe sound theoretical grounds for not reacting to traffic you cant see, however if the traffic is getting closer and is the same height adjusting to ensure vertical seperation has in my experience always avoided a conflict whether or not I could see the traffic.
It is interesting to note on how many occasions you never see the traffic or when you do just how close it is. Knowing the quarter in which to look to spot the traffic is a benefit but I am not convinced the benefit is as significant as you might believe.
The cheaper PCAS unit is self contained. It runs on two AA batteries for at least four hours. There are no trailing wires. It is easy to stick to almost anything. The more expensive unit does require an external power source but it is easy to connect a small battery pack.
All these systems rely on the other aircraft transponding. I am constantly surprised by how many are, particularly the higher you are and the worse the weather. You are inevitably most at risk low level on a sunny day!
In my view these systems significantly reduce the risk of a mid air. They are not a substiute for maintaining a good look out nor do they detract in any way from doing so.