The web site is already operational and the Android app is already on the Google Play store. The iOS app still hasn’t hit the App Store.
The 1080p 60fps video quality is incredible (watching a Heroes of the Storm stream). The chat is kind of so-so though (they do have their own emoticons) and will need to develop its own identity (though a lot of Twitch-ism is probably going to be used by viewers regardless).
But the site is far more nicer than I expected when it comes to viewing. On the streaming side, until both PS4 and Xbox One have YouTube Gaming integrated, streaming is going to be limited to desktop games or those who already stream console games using something like an Elgato (the big market though is console game streaming).
Update: the mobile app is really nice and offers a far better user experience than Twitch. Again, the chat functionality looks and feels clunky but everything else is a major improvement over Twitch since it leverages on what Google does best when it comes to search as well as YouTube’s video on demand aspects.