With Google Voice, you sign up and receive a phone number. To set it up on your phone, follow the on screen prompts via the web that walk you through the sign-up process. You'll be asked to call a number which will provision your phone to forward the calls to the new google voice number.
People can still call your sprint number and you'll receive the call like normal. Except, when they get your voicemail they'll hear, "the google voice subscriber is unavailable..." you can customize the greeting like any other voicemail.
You can then download the google voice app to your phone and manage the voicemail. You'll have the option to be alerted via email, text message, or both. Voicemails are transcribed (if the message if clear) and will be emailed to you.
You don't ever have to give out your new google voice number unless you want to. People usually use it as a second number for business related things, or forums sales through craigslist, etc. Basically, if you don't want someone to have your personal number, then give them the google voice number. That's how I think of it. There is a video on the google voice page that shows you a little about the service. check it out and see if it's for you. Whenever you switch ROMs you'll just have to download the voice app and complete the setup to get it up and running again. Nothing compared to the sprint app workarounds.