There can't be much new to be written about Google Voice. Everybody has covered it. Here's the wikipedia article on it and here's the NY Times article on it.
I've been using what is now Google Voice since it was GrandCentral. Google bought GrandCentral and seemed to let it lie dormant for a long time. In the Spring of 2009 Google renamed it to Google Voice and began ever so slowly handing out new invitations. Google killed off a couple of GrandCentral capabilities (being able to make your ring-in sound like a European phone) but added many more such as voice mail transcription (cute but really pretty worthless).
I use it for 2 purposes. First, I have it ring my home phone and my cell phone simultaneously so I can always be found. I can even add locations when I'm going to be somewhere else, e.g. my mother's, for a while. That's so easy to do.
Second, I use Google Voice to present a consistent caller ID. The free long distance is nice but I use Google Voice to make even local calls so that people I call always see the call coming from the same number.
There are some other considerations. If you receive a call on your cell phone through your Google Voice number you don't get the benefit of mobile-to-mobile. Also, Google Voice is subject to service disruptions like Google is. Occasionally, you'll notice things like incoming calls going straight to voice mail or even that the call can't be connected.
Google Voice is not a part of Google Hosted Apps. What this means is that if you have a Google Hosted Apps e-mail (like I do) that your contacts are not shared between your Google Hosted Apps e-mail and your Google Voice. Maybe someday.
I'm sure that Google is going to continue to develop Google Voice. This service will redefine what voice service delivers.