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.
Palin was picked by McCain because there would have been an open revolt otherwise.
Remember that McCain was going to pick Joe Lieberman as his running mate until the GOP base revolted and he backed off. That's when he went with Palin. I don't think people would have held their noses for Joementum, they would have stayed home.
My opinion is that there were a lot of Democrats who would have pulled the lever for McCain over Obama, especially those hard-core Hillary supporters, who were still bitter with the way the nomination process went in 2008. But Palin, basically made them leap into the arms of Obama in the end, although she did fire up the hard-core conservatives.
That was certainly the opinion of Charles Krauthammer ("The Palin selection completely undercuts the argument about Obama's inexperience and readiness to lead.... To gratuitously undercut the remarkably successful 'Is he ready to lead' line of attack seems near suicidal."), David Frum ("The longer I think about it, the less well this selection sits with me.... If it were your decision, and you were putting your country first, would you put an untested small-town mayor a heartbeat away from the presidency?"), and Peggy Noonan ("It's over... the most qualified? No.").
As for Palin "making them leap into the arms of Obama," I'd say that she was the target of opportunity of the MSM rather than anything she herself overtly did. From the Charlie Gibson interview as he looked down his nose at her while asking if she knew what the "Bush Doctrine" was, to Katie Couric asking her what she reads, it was a coordinated agitprop effort to turn voters away from the enthusiasm that Palin generated in the GOP base.
To the critics that said she didn't have the experience, I'd ask to compare her to Paul Ryan, who was Romney's pick four years later. At the time, Ryan was a six-term Representative from Wisconsin (from a district with about 700,000 people, Kenosha and Racine), elected in 1999. Ryan didn't have any leadership roles until after he was Romney's running-mate, except for chairing the House Budget Committee since 2011. Prior to joining Congress, Ryan held various aide positions for Republicans.
Contrasting to Palin, she first ran for office in 1992 for Wasilla City Council and won a four-year seat (Ryan was working as a waiter, a fitness trainer, and other odd jobs). In 1996, Palin ran for mayor of Wasilla and won two three-year terms (Ryan became a speech-writer for VP candidate Jack Kemp, and ran for Congress while Palin was running for her second term as mayor). Palin then ran in 2003 for the Chair of the Alaska Oil and Gas Conservation Commission and won that two-year seat (in 2000, the first of only two Ryan bills to become law was passed, renaming a post office in his district). In 2006, Palin became the ninth Governor of Alaska (the other Ryan bill to become law was passed in 2008, lowering the excise tax on arrow shafts).
I would say that Palin had the clearly stronger resume than Paul Ryan, but Ryan didn't get the same treatment that Palin got.
It's obvious by contrast that the establishment feared what Sarah Palin represented and made it a mission to take her out. It's also what they're doing to President Trump now.
And Paul Ryan became a Never-Trump backstabbing RINO who sabotaged the GOP's chances to hold the House in 2018.
-PJ