One of the few gripes that many otherwise happy Google Voice customers have had is that you have to get yourself a new phone number if you want to use it. This is old news now, as number porting has now been enabled for Google Voice to a limited number of users. If your number is portable, the service will cost you $20.
The service doesn't come without a slew of warnings, though: early termination fees from your current cell provider could make this an expensive process. Android phones should play fairly nicely with this due to Google Voice's deep integration with Android, but this may be more of a decision for iPhone fans. But hey, cutting ties from your carrier may just be worth it as aside from some occasional downtime issues experienced a few months back, Google Voice's user experience has generally been a positive one.