As a longtime AirPlay user, I find this less exciting since AirPlay works with pretty much anything that's on your iDevice. That said, cross platform is interesting.
And Google Cast, with its lovely "You may not publicly distribute or ship your Google Cast application without written permission from Google" warning, isn't?
I hope the protocol used is less buggy than AirPlay. AirPlay is very shaky with many apps. I am not sure though if it's the protocol's problem or the app's.
