Voice quality over 3G and Wi-Fi is excellent.
The major problem with the app is that it does not run in background. Each time you switch out and return to it, you have to log back in. Worse yet, you cannot receive calls unless the app is in the foreground.
Integration with iPhone contact list is handy. However, the network switch and/or app are not smart enough to strip out call prefixes and other characters from phone numbers such as "+65" for Singapore calls, "(" and "-". It requires you to enter "008" or "018" for international calls. You have to edit yourself every time you make a new call which is a big hassle. Since the service is called "GLOBAL Digital Voice," it should be friendlier to people who have set up their contacts list to function globally.