I bought my dad a Chromecast Audio so he can be the guinea pig. My understanding is you'll be able to cluster multiples of them into playing the same thing across various rooms of the house. Quite looking forward to thumping industrial 24/7 on those things.
I'm waiting for the "party mode" to be enabled to see if it actually works. Currently it isn't enabled and without it you can't run two or more devices at the same time with the same signal. It was supposed to be out "in December", but I can't find anything other than the promises so far, no hard date and no other info on it (as of last week anyway).
I have my AV system, my whole house audio (5 rooms), and then several Bluetooth speakers and I want to be able to feed the same signal to each from a phone/tablet without having to leave the phone/tablet on and in range. Previously my only real choice was to get Sonos components to do this, but at a cost of $350 per unit (would need 2 units) plus replacing all my BT speakers with Sonos speakers ($250-$750 each) it was too expensive of a solution. Chromecast audio at $35 per unit is ideal, but without party mode (all units playing same source controlled by one device), it is worthless to me.
I look forward to seeing if it works. I got a Yamaha receiver for my home theater recently and it has MusicCast, which is REALLY slick, but I read the MusicCast devices outside the receivers lose sync too much and when using multiple devices simultaneously pretty much impossible to listen to. Shame, I like the interface on the tablets for MusicCast.