Palin is a much higher risk/higher reward than Biden. McCain prob. needed to roll the dice on something like this, because he wasn't going to win the election with Romney or Pawlenty.
Maybe Palin can slice off some Hillary voters, I dunno. Being pro-life prob. isn't going to help a whole lot, and being a creationist scares away moderate/independents.
But she does have a pretty interesting story, so maybe that snags a few more people.
Personally, I think it isn't going to win the election for John McCain. Palin has a downright frightfully thin resume (even when compared to Obama). You might believe that Palin may help Mccain win the election...but I don't see how anybody can really be comfortable with Palin as the actual PRESIDENT, which could happen. Biden is absolutely qualified.