The latest info I have, sales of the Flarm unit are now over 11,000 units.
We are all looking to improve safety for all, my vote goes to the Flarm system, capable of being fitted into flying machines with limited power supplies and payload.
Using a barometric and GPS based altitude processor, the error will be much less than a transponder based sytems with the possible errors on the static pressure line. Mode C is +/- 200ft.
I am sure Flarm design teams are working hard on certification so our colleagues with more sopisticated aircraft can install such a system at a low cost.