Google Voice is a new version of the GrandCentral service it acquired in 2007. Exisiting GrandCentral users now have access to the service. Google plans to offer it to the public in the near future.
The transcription feature is an advantage when checking your messages. Google Voice emails the transcripted text messages and makes it easy to search. The caller's number is spotlighted. On the Apple iPhone or T-Mobile G1, the phone number is highlighted in the email so you can click it to call back.