I know allot of people are happy with AT&T. I saw a poll on this site last week that asked if people would switch and there was a huge amount that said no, they would stay with AT&T.
Another site I was on showed a large number of people willing to move. It all depends on the audience.
Several polls I've read show about 23% would migrate. Of course perhaps not all of them would actually follow through. It's one thing to answer in a poll that you would and another to actually follow through.