Just how popular is Google Voice, you ask? In its reply to federal regulators over the Apple-AT&T-Google Voice flap, Google stated it has 1.419 million Google Voice users, and about 570,000 of them are using it full-time, seven days a week.
As an interesting side note, BusinessWeek apparently gleaned the number of Google Voice users from a PDF that was impropertly formatted, desipte the number of users appearing to be redacted. Oops. (So not to be confused, we're talking the number of users, not users' numbers.)
Also noted is that Google Voice may eventually go global (it's still only available in the U.S.).
BusinessWeek via Cnet