Unfortunately the see and be seen environment has a lot of limitations - fundamentally on the performance of the Mark I eyeball, so having something that alerts you to most traffic is almost surely a net improvement. Of course having the technology cheap, light, and low power enough to be in the vast majority of aircraft would be better.
So, that would be Flarm, then?
Had to put one in the glider to fly in France, the FFVV mandated it and then subsidised it. Paid about 600 euros over three years. Of course, nearly everyone needs to have it before it is really useful.