Jonathan Chait pulls out all the research from political scientists so that I don't have to, to show that, yes, Sarah Palin was almost certainly a drag on John McCain in 2008.
One thing I'd add to Chait's takedown of Matthew Continetti, who argues that Palin's selection energized the GOP base. The missing piece there is opportunity cost. Yes, had McCain selected (pro-choice Democrat) Joe Lieberman, the base would have been further demoralized. But that wasn't a realistic possibility. No, McCain, who was always eyed suspiciously by conservatives, had little choice but to select someone who conservatives would be happy with. Indeed, if there was one thing in summer 2008 that was pretty inevitable, it was that as the primaries receded and the prospect of a Barack Obama presidency started to loom large, the conservative base would find some reason to get enthusiastic about the McCain-Whoever ticket. If it wasn't Palin, it would have been something else.
And for everyone else, Sarah Palin in 2008 was a clear negative.