See my comments on the thread: GA ADS-B Trial in Southern England.
In particular, on 9/8/15, I wrote:
I increasingly do not look much at the screen but certainly pay attention when the unit starts to bleep, now through my headset. This usually means Mode C traffic is near so I have a good look out the window and waggle my wings. This is particularly useful when flying long distances in apparently empty airspace in Scotland/Northern England but encountering other traffic not known about by ATSUs or warned about very late (Of course, under basic service, ATSU have no obligation to warn me about conflicting traffic anyway). The unit also annoyingly bleeps briefly when you are on short finals and there is a transponding aircraft sitting short of the threshold but I suppose this confirms it is working.
I am not sure of the value of displaying traffic on a virtual radar screen (on Skydemon etc). You really do not need to know about most of the traffic nearby- only about the ones that might hit you! You should be looking out the window if traffic is nearby, not at a computer screen. That is why increasingly urgent warning bleeps are so useful.
.....
So, despite the fact that lots of traffic does not have a transponder, FLARM or ADS-B out, PowerFlarm is definitely worth having.