Lets see, a free telephone booth, at public places like Airports and Universities, I can imagine ONE person talking on that thing for days. People are rather inconsiderate. At least in old phone booths you had to keep inserting change every 5 minutes or something (been a LONG time).
That said, this is nifty, but its only free for now. Obviously, Google can't keep this up forever, how will they make money off your phone calls, listen in and interrupt you ever few minutes with an ad? That would never fly. I imagine it will change over to skype-like prices. In the end, I find MagicJack a lot easier, since I can connect my phone directly to it. I don't have to sit in front of my computer or lug my laptop around the house if I want to make a call and move around.