How would you even measure Airplay success? It is not a stand alone device. It is in hundreds, if not thousands, of third party products. When I was searching for new receivers last year, it was part of my criteria for choosing one. Luckily, it is included in nearly every receiver out there, so it was no problem finding it. It is also in AppleTV2, 3, and 4. It is in Airport Express devices. It is in tons of third party stand alone devices. It is in speakers. I would call it successful if you consider market saturation important.
Just a quick list of some of the third parties that utilize Airplay:
Denon, Onkyo, Sony, Phillips, McIntosh, Marantz, Phillips, Pioneer, Harmon Kardon, Yamaha, Classe' Audio, NAD, etc.