Lifehacker.com suggests GoVoice. I have downloaded it to use for texting and retrieving voicemails with it, but my phone is not activated yet, so I have not set it up for calling. One thing I dont like is that the live tile apparently only goes up to 9 notifications, then it says 9+.
I guess it depends on why you want to use Google Voice. I use it so that I can have an integrated inbox that consolidates my voice mails, emails, text messages, and chat logs. I also enjoy the ability to send and receive texts from a computer, manage voicemail recordings online (play, archive, forward, etc), and the ability to control which of my phones ring when, per each caller.
If you want to get out of paying for your phone plan, but still be able to make calls, Google Voice may not be for you. You used to be able to put your secret Google Voice number in your MyCircle or similar program, but nowadays I believe Google calls a different number every time you dial out. I hear there are ways to get your iPhone to cheat and use Google Voice entirely though Google's Google Talk servers, but I don't know the details. It sounds sketchy.