Hi, I've been lucky enough to get in on the beta, unfortunately I only have a handful of iphone users that I can call.
For those comparing it to Skype, I personally think the Android Skype app is horrible. Viber has been implemented in a much slicker manner.
The great thing is that I don't need an account per se, it uses my phone number and the numbers of my contacts to find who I can call/message with it. Very easy, no logging in etc required.
I see the latest version has been integrated with the native Android contact list? The problem is that those of us that use alternatives (I use Contapps), Don't see a benefit from that. I would suggest working with the Devs of other Contact applications to try and get it integrated as seamlessly as possible, across as wider range of apps as possible. That could really be the key to the success of Viber on Android.
I'll echo what others have said (that you've no doubt heard a thousand times) and suggest that you need to develop something for PC/Mac or a webapp quick-smart. I live half a world away from most of my friends and family, most of whom don't own smart phones. To turn it into something I use all the time there needs to be a way to connect with those people, perhaps a login via facebook or something?
I know you don't like the comparison, but Skype dropped the ball a while ago and you now have a chance to pick it up.
You'd best do so before Google decides to make Google Voice available outside of the US.
Congrats on a nice app.