I've been using Grandcentral for some time now and love it. By adding my Grandcentral number to My Circle and only giving that number out to people as my phone number, I essentially have unlimited free incoming calls now. My family uses my Alltel number and that way I can take advantage of the M2M and unlimited text messaging and since they're the only ones I text anyway it works out perfectly.
Since My Circle was implemented and I added on Grandcentral I was able to lower my Alltel anytime minutes from well in excess of 2000 minutes per month to an average of around 200 - 300 minutes per. With 3 lines of service plus a data card that saved me a nice little chunk of change. Consider me sold.
Since Google bought Grandcentral a few weeks ago you have to be invited (like Gmail) to get an account and I've given out all my invites it's become so popular.
By the way, Grandcentral is free and also allows you to set individual greetings. The fact that Google now owns it is a testament to how well it works IMO and also a good sign it will continue to grow and develop.
Just my 2¢ worth.