I have and use Google Voice. I really like it. I don't use it to the full potential it has, but I do use it quite a bit. Usually on forms or something I will give my Google Voice number, keeping my cell a little more confidential.
What I use it most for is the Voicemail abilities. I have unanswered calls for my cell forwarded to Google Voice. Voice then records it, transcribes it and sends it as a text message to me. It is also sent to my Gmail account, where you can use the Gmail player to listen to it. Very neat. I also use the Chrome Extension for it a bit. Lets you SMS someone from your browser and I really like that.
It has some cool features, and I hope more are on the way. I'd like to be able to select specific phone numbers only be allowed to ring to my cell phone during specific hours of the day. Otherwise just go to voicemail. That'd be way cool.
I also use Voice occasionally to make calls to numbers I don't entirely trust. The Chrome extension makes it way easy to do so as well. Every phone number on any web page is turned into a link. Click it and it will give you the option to call that number from your Voice account.
I have never used it to make out of country calls, as I never make out of country calls. Good prices though, from what I understand.
I don't however have any available invites. Sorry. I am sure someone will have one.