GV mobile is nice...but it's really far less than ideal. I've also had it be hit or miss in terms of stability (about 1 in 4 calls placed from GV Mobile never actually go through, in my (admittedly) limited experience).
GV mobile is an app created by a third-party. And, quite frankly, compared to the "official" app (guessing based upon the BlackBerry version) it PALES in comparison. I find GV Mobile on my iPhone (I got it before it was pulled from the App Store) to be almost completely useless. Google Voice on my BlackBerry is useFUL.
Yes, but the official Google Voice app* (or even the 3rd party ones that were kicked out) do NOT use VOIP for their connection. Which was the whole point.
* - I'm extrapolating on the "official" iPhone app, since it doesn't exist, but that's how the BlackBerry app works
As has been explained - Google Voice may use VOIP technology on its backend, but users do not interact with it via VOIP - it's all "regular" POTS or cell network, so, again, it may use VOIP internally, but it's not VOIP from the end-user, so yes, you still use just as many AT&T minutes.