Not the least of them is it's voicemail option. When you reject a call on your Google Voice number, your caller is able to leave a message to you like any other voicemail. But the sound file gets sent to your Gmail account along with a (not perfect) transcription of the message.
But if you're not ready to print up new business cards, the service does you no good! Luckily, Lifehacker comes to the rescue with a simple but brilliant idea:
There are codes you can punch into nearly any cell phone to make it forward calls that you don't pick up because you're busy, out of range or powered off, or simply not answering, to another number. Make that second number your Google Voice number and set Google Voice to "Do Not Disturb," and it'll pick up the call forward instantly, just like your primary voicemail.
But be warned: Many cell service carriers have a limit on call forwarding minutes. You could risk a big bill if you don't do a little research on your specific carrier and plan!