I have been a GrandCentral user for a long time, way before Google bought them to create Google Voice. I love the service, and it got better with time. Slowly, but it got better.
[START RANT] I am just waiting for the possibility of sending international SMSs to call it the best tool I own (oops, use as a service). Not sure why international SMS support is not there, because I would pay for it! Nowadays, I have to open up Skype and send an SMS to an international friend (paying dollars), to get an answer in Google Voice, then go back to Skype. Aaarrggghhh!!! [END RANT]
Rant excluded, I think Google Voice is awesome. And I do not believe people understand how critical it is going to be for Google domination in mobile.
First of all, if you have Google Voice, you do not need one particular carrier. You need someone to give you connectivity, but you are totally unplugged from them. You can move to a different carrier and not even notice it, since your number is on Google Voice. It makes the carrier a voice and SMS pipe. You can simply go with whomever gives you better connectivity and lower prices. Huge.
Second: the future of computing are a bunch of devices you carry with you, all with some wireless connectivity. It can be your tablet, your smartphone or your laptop. They will have all-the-time connectivity one day. 4G or 5G, whatever. They will be always-on and able to talk to the world. Still, you will want a single number, one that people dial to call you. They call you, the smartphone rings, or the tablet rings, or the laptop rings (in the last two cases, you'll need a Bluetooth headset if you are in public). Or all of them at the same time. It does not matter.
What matters is that you will need an infrastructure to make all your devices converge into one for the outside world. It is like email and the ability to read it from different devices, but real time. You do not give people three different email addresses, for when you are home, work or in between.
If you have an Android tablet, it will all work through Google Voice. If you have an iPad, too bad, Apple does not have it (actually, they will try to prevent it to limit Google and help the carriers).
Eventually, more people will be forced to choose Android over iPad. For a stupid feature, but a very important one: nobody wants to give the rest of the world two or three different numbers and say "try the first number, then the second, then the third". There is a reason why nobody calls you at home and work anymore: they call you always on your cellphone because they know it is the only number they have to dial (even if it might cost them 10 times more, as in Europe). Convenience is everything. People are fundamentally lazy.
Somehow, Google Voice is key for Google domination in connected devices. Weird.