I've used VOIP over both EVDO and HSPA. Sometimes it is good enough, but not always. I haven't hesitated to call Mom from overseas this way, but I wouldn't call a business client. When it works well, you are basically stationary. Walking around a hotel room is fine, riding on a train is usually not.

My understanding is that GoGo basically ties into the ground based cellular network using multi channel EVDO connections so I assume the connection would be similar.