While Chrome plays local media files fine for example and also YouTube videos and videos from several other high profile services such as Netflix, you need to install add-ons or apps for other services that are not supported yet out of the box.
As long as the Chromecast sucks on mobile devices, then it will be a failure to me. It's only useful for casting a computer to the TV… I rather connect my Mac directly to the TV then.. It's easy.
I need full screen mirroring from a mobile, and also able to cast documents. As long as that is not possible, the Apple TV is a much better buy.