Google Voice is a great service and now you can easily use it with your iPhone. iPhone users can access Google Voice through a new website that almost entirely mimics an actual application.
Your phone number will be displayed as your Google number on caller IDs. If you are a solo practitioner, this is nice way to prevent your personal number from being displayed (although you can also easily turn off your caller ID on an iPhone). Google Voice will also send free texts through your phone. Lastly, the service will sync up with your existing Google contacts.
The biggest downside is that the although Google Voice is accessed through a website, it still uses your AT&T minutes to make calls. The website does not know how to dig into your contacts, so unless your iPhone contacts are synced with Google, you cannot access them through the new Voice site.
Definitely an improvement. Even if you use your AT&T minutes you can easily record a call, and call from your Voice number. If your voice number is your work number, that is a big plus.