This will be more of a fallback in the event that Google Voice ever drops the ball for me. GVoice's seamless integration into the default calling process on Android devices makes it a much more attractive option than Skype.
I'm hoping Google gets their act together integrating Gizmo with Google Voice, but until they do this is a great stopgap measure. In the long term I'd be much more comfortable relying on open protocols/apps (SIP and Sipdroid) than Skype though; they have been abusive in the past and even pulled their app for a time.
I kind of almost made a call once this way.