As Nicole said, the drawback of Google Voice is the lack of MMS support, but this is only if one plans on texting pictures. Trust me, if I never needed to send or receive pics, I'd never have to give out my phone's number assigned to it by the carrier. The only time I use that number is for MMS messages if the other person doesn't have Whatsapp Messenger installed. If you're on Sprint, as she said, your GV number is your number from the carrier, as well, so it will work.
Now, when using GV from a non-smartphone, it's a little trickier than just dialing. You have to dial your GV number from a phone linked to your GV account (home, cell, whatever), and then dial the number you want to call. The person on the other end will see your GV number on their caller ID, not the number of whatever phone you are calling from. The other option is to dial from the Google Voice webpage and it will ring your phone and when you pick up dial the number on its end that you are calling.
From smartphones, you can get the Google Voice app, and use its settings to choose to use GV for all calls, only international, or having the phone ask you when you dial out to use the phone's carrier-based number or your GV number. At least this is how it works on Android phones. I cannot verify how it works on iPhones, Blackberries or Windows Phone 7 phones.
Also, as Madjohnson said, you could also use Google Talk to dial out from your computer. If you don't have a GV number associated with your Gmail account, then it will be a random number from Google's switchboard (basically) that they see. But, I would suggest a headset with a long cord if you wanted to try this. If you do have a Google Voice account, then it will show that number when you call.