Surely, someone has asked people in other countries what their experiences are with their iPhones on their carriers. Given that the rest of the world is on GSM (Korea excluded), we should pretty well know whose fault it is.
Last week I visited mountain view for Campfire One. In the three days I was there, I had two calls dropped on me. One of them was dropped while walkig around outside at the Googleplex. My signal often wavere between zero or two bars and five bars for no explicable reason. I would occasionally lose signal entirely inside buildings (my hotel room for one).
In my experience, a perfectly working iPhone in Canada becomes unreliable in California. Take from that what you will.