I don’t yet use Google Voice as much as I could, but have to say, the service is pretty incredible. My need for a second number, much less a primary phone number, is really not that great; I only do business via email and have for years. It’s one of the ways I keep my freelance time from intruding on my full time employment and rightly so.
From Google Voice’s introductory email you can get an idea of some of the things it can do for you —
Welcome to Google Voice. Google Voice gives you a single phone number that rings all of your phones, saves your voicemail online, and transcribes your voicemail to text. Other cool features include the ability to listen in on messages while they're being left, block unwanted callers, and make cheap international calls. We hope you enjoy using Google Voice.
If you need a secondary phone number, or a means of filtering your calls for business, this is a free solution.
Want to transcribe your notes? Setup a Google Voice account and call yourself. Voilà! It’s free, too.