Just to give my 2 cents..
I've been using viber for months now..
if i'm on a strong wifi & my buddy on weak digi 3G.. the call quality sucks.
if i'm on strong digi 3g & my buddy on maxis 3g the call is good.. no lag/delay
however i'm using the YES trial now & i;ve gotta say its pretty good. reason? because my buddy doesnt have to be on any data plan..
as long as i'm on my own wifi i can call based on 9cent a minute..
good value cuz Digi at my house is very bad (only one line no 3G)