GV is actually quite awesome. I got it with the understanding that it was a replacement for my voicemail inbox. It could text, email, or play and transcribe into text your voicemails without having to call yourself.
What I found out after using it, is it is also a forwarding service, and can link all your phones from one number that you can choose, allows international calling at incredibly low rates, completely free SMS text messaging, and more to boot.
People have used their GV numbers into their Friends & Family plans to allow calling to anyone without using their minutes, lowered if not removed their text messaging plans. It's literally all I use now.
Google Voice Help offers a pretty good FAQ about some of the things it does, as well as a number of threads in this forum (if you don't mind sifting through thousands of invite requests) that can help give you a better understanding on this truly amazing tool.
Follow the link from your invite email (hopefully set up with the GMail account you use on your Droid to easily sync it) and set up your very own Google number, d/l the Google Voice app from the market if you don't already have it, and enjoy its wonders