As a RingCentral customer, I've had most, maybe all the important features of for three or four years. And I have features--like multiple local numbers in different calling areas--that I haven't seen mentioned for Google's service.
What's the difference between the two services? Price: I pay about $200 a year for RingCentral, while will be free, at least in the beginning. There are also important feature differences that I'll describe in a moment.
Pricing alone makes Google attractive for me as a home user, but having an 800-number, plus local numbers in several cities, makes RingCentral worth paying a premium for in my business.
RingCentral is also selling multi-line systems, allowing a virtual company to use a single virtual PBX system. This further distances RingCentral from Google Voice, at least for the time being.
I have not actually used Google Voice, except as a caller. My invitation has yet to be sent, though I work with people who already use the service and are extremely happy with it. Much as I am extremely happy with RingCentral.