With hindsight, did John McCain make a mistake in picking Sarah Palin as his vice-presidential candidate, all those years (sorry, months) ago? I don't find this question easy to answer. On the plus side it injected excitement and dynamism into a campaign that seemed in danger of running out of steam. Most importantly, it had an electrifying effect on the party's core supporters, who started organising and persuading for the campaign with far greater enthusiasm once Palin was on board.
On the down side, she eventually became, post-Couric etc, a net negative for the campaign in the polls, as more people came to believe she was unqualified for the job and that this reflected badly on the man who picked her. Her selection also had the effect of angering or emboldening key figures who, already uneasy with McCain, might otherwise have stayed silent or even endorsed him (eg Powell).
A less obvious factor, but perhaps the one that swings the balance against her selection, is how the Democrats reacted. She fired up the GOP base,yes, but she also fired up the Democratic base. As Obama campaign manager David Plouffe remarked the other day:
"She was our best fundraiser and organizer in the fall."