From
http://www.how2soar.de/index.php/led-flash-calgnome/649-zur-sichtbarkeit-von-led-blitzern-2
"None of the LED-flashes (the Schleicher flash in the tail fin, the Glider Flash, or the flash described in these pages, not even the strobe lights of a jet 10 km away) will catch your eye in bright sunlight.
However, when your FLARM screams and you try to find the plane in the direction advised, then you will see the flash immediately, even before you can detect the contours of the plane. That is exactly what the flash is made for."
So this is marginally effective if both planes do not have Flarm. Seems like it would be good when flying under cloud streets, but you'd want to have it on all the time for planes that do not have Flarm to trigger the flash.