I use Google Voice in a very limited fashion because since I'm on Sprint with its any mobile, any time, it makes Google Voice not that great a deal for mobile to mobile calls. So, unless Sprint sees the Google Voice number as a mobile phone, then it's going to count against my airtime minutes when I get a forwarded mobile caller from Google Voice, or I use Google Voice for an outgoing call to another mobile. At least that's the way I comprehend it.
As far as voicemail, well, Google Voice is better than VVM in my opinion since I can have different announcements for callers. And, when I set a caller to be blocked, Google Voicemail answers with the three tones and states that the number has been disconnected.
However, I admit that am a bit excited and patiently waiting to see what Google is planning to roll out of their labs since their acquisition of Gizmo5.