I must saythat FLARM does appear to fit all of the requirements admirably.
Cheap, effective, reasonably accurate, standalone device, interchangeable from aircraft to aircraft, and which also has the advantage of operating outside the radar and ATC environment.
This last point might be quite an incentive for those pilots who are apprehensive about sqawking their location to ATC in these days of compulsory infringement reporting.
Although I operate with a transponder I would happily buy a FLARM unit if there were substantial uptake. I wonder what could be done to "make it happen?"