Ask any glider pilot flying around in the Alps about the value of FLARM and you will get a unanimous answer. In a sport where mid-air collisions are one of the most popular ways to die, people gladly spend hundreds of pounds and make room on their panel and in the glider for the equipment and batteries.
There aren't enough mid-air collisions in power planes to get people interested in collision avoidance. In my experience, power pilots are more worried about engine failures or crashing in to a hillside in bad weather, probably because that's the most popular way to die in a power plane. If you could make a box that helped prevent engine failures or CFIT people would be snapping them up in their thousands.