From Google Operating System:
Google Voice, which provides people with a single phone number that can be used to reach them on their work, home, or cell phones, has 1.419 million users.
Google Voice charges only for outbound calls to international locations; at present, fewer than 4% of all Google Voice users place outbound international calls.
Google emphasizes that Google Voice is not a phone service, it's a Web-based software application.
I have used Google Voice for about 2-3 years, when it was still a service called Grand Central, and my experience has been positive. You can embed a Google Voice widget in your blog sidebar which lets the website visitors call you without revealing your or their phone number. Google Voice automatically transcribes the voicemail as text and emails you a copy.
Nexus One - Google Voice