When I was in Friedrichshafen in April, I asked the Flarm guys how many they had sold so far. Approx 13,000 in Europe. Mostly to glider owners. Some to aeroplane owners - they thought about 10-15% I recall. There are approximately 22,000 gliders (aircraft, not people) in Europe, both EASA controlled in terms of airworthiness and non-EASA, i.e. Annex II. Data from European Gliding Union survey of member countries, which I was involved with. That means approximately 50% of gliders in the whole of Europe now have Flarm, if the numbers are reasonably correct. In Germany, with about 8,000 gliders, I am led to believe it is a high %, and also in France particularly in the Alps. Flarm is about 5 years old as a product on the market.
In the UK the reason for low numbers with Flarm so far is that until last October, we had not got clearance for use of the relevant radio spectrum. Until that was resolved - the BGA's safety case won the argument quickly and the use of the spectrum is licence-free - UK glider owners were inevitably putting off the purchase of what they saw, for the main part, as a sensible option. I suspect the take-up rate this year will increase considerably. And it doesn't require an EASA mod approval.