No, I don't use Viber.
I think that this system is useful for iPhone, Android and similar systems for text, picture, video and voice communication - especially for professional purposes:
https://en.wikipedia.org/wiki/Viber
But being permanently "connected" everywhere at any time is not so good, especially, when, for instance, families, friends or couples, instead of sharing a good time together, are constantly interrupted by messages and calls.