I've been using YouMail and it works well. Only complaints I have are that it complains about not having access to the internet after being on a call for longer than 30 sec, and that it plays messages through the speakerphone instead of handset so privacy is a bit of an issue.
I never thought about using Google Voice as voicemail, even though I've had it for quite some time, but this is an awesome idea. I'd love to have transcription and would be great if I don't keep getting the "internet connection not available" message from YouMail.
I think I will switch now!