Google Voice is a very interesting service. If you were one of the people (like myself) that got an account on GrandCentral.com before they were bought out by Google, you are now eligible to be part of the Google Voice beta.
It offers a lot of interesting services such as visual voicemail, speech to text, VOIP, free long distance, and many others. However, in order use most of these, you need to use the phone number Google assigns you. Google can’t be your voicemail provider unless all of your calls are routed through them first.