i'm using it now, tested the text messaging with an iphone friend, works perfectly. have not tried calling yet tho.

in theory yes. however these apps like whatsapp and viber will definitely eat more battery than a normal/typical application because it sits in your background and most likely does some data connections off and on to the servers. but i think it is not that noticeable. obviously if you're on wifi or 3g it will eat more battery. in any case battery life will surely be affected but not too significant.

dont worry, you can chat with your iphone friends who are using viber too, it is cross compatible!