I have experimented with Skype, Gizmo, and GrandCentral. The quality of each was essentially perfect as long as I had a good data rate on the WiFi connection. Once I called the United States with GrandCentral from overseas with an extremely low data and the conversation was "choppy", but we were still able to communicate.

For calls to the United States, GrandCentral is a GREAT service if you can get it. EVERYTHING is free when calling to the United States and when receiving calls (from anywhere) on your own free phone number. I made calls to the United States from the Greek Islands and everyone said that it sounded like I was "just next door". You cannot call to locations outside of the United States with GrandCentral (although that may be changing).

I bought $10 worth of time on Skype so that I could call any place in the world. I tested it with calls to several countries, and it worked perfectly. Of course, you alway have to figure out the country code etc. when calling another country. Skype minutes expire unless you use them at least once every six months. It's probably just a matter of time before I lose my $10 because I always use GrandCentral.

I have found that I can get free WiFi in any city in the world that has broadband. I simply walk through a residential neighborhood and it usually doesn't take more than a few minutes to find an unprotected WiFi connection that works. It works great for a quick email exchange and a GrandCentral phone call.