For me this level of integration was always brilliant. Mind you, that at the beginning Google Talk was tightly integrated with Android (it was 2.x AFAIR), you could see people's online status in address book, start chat from there and so on. Sadly with newer Android versions and switch to "better" Hangouts it is no more integrated with system in any way. In fact it is not that obvious in Hangouts application itself who is online and who is not. Strange, perhaps Google did this due to GSM operators lobby which saw danger for their paid voice and text services... As usual - money are more important than pushing our world forward :-(