Without Palin, McCain would have had almost no conservatives voting for him. I was disgusted at the thought of voting for McCain. I was convinced that he was going to select Joe Lieberman as his running mate (I do believe he wanted to). Had he done so, there is no way I would have voted for him.
Looking back, I don't think it made much difference. McCain became even more loathsome in the years since the election.
I think in the end there were more driven towards voting for Obama, than those who were convinced to vote for McCain, because of Palin.
Assuming McCain had a different running-mate, even so, I still think most conservatives would have held their noses than face the alternative of having Obama in the White House.