Honestly, better than most of the other alt-2008 tickets this board has seen!
She has a good chance of cleaning Joey B's clock in the VP debate, and has a better shot than Palin did of actually picking up some votes as an interesting bottom half. She doesn't alienate people the way a Lieberman pick would, but she's maverick enough in her politics to scoop up some moderates.
She's a blank slate on economic issues and can certainly talk the McCain line on recovery. She's publicly against the GOP party line on abortion, though, and McCain won't be able to count on the Christian Right in this election.
The Bush association unavoidably tarnishes her, and the ticket still goes down in flames. Still, not the "worse than OTL" you usually see with these scenarios.