Maybe we could ask the Dutch what their experience is as Mode-S is already mandatory there.
My guess is that a gaggle of 15 - 20 gliders all circling in a thermal, with constant variations in relative height, speed and direction is going to provide the technology with quite a challenge. Wonder how the problem of which glider should transpond for everybody would work ? "Glider XYZ please stop transponding as you're near to somebody else who's trasnponding already"