Having spent two summers up in Alaska I knew how well liked Gov. Palin was and see McCain's choice for VP as picking a VP candidate, not a VP. She helps him get elected, but I don't think she will be of much help in governing. This in contrast of course to Sen. Biden who will be a big asset if Obama becomes president, but is less helpful as a running mate.
Palin shores up social conservatives and theoretically peels of Hillary die-hards, appeals to Westerners, and masks McCain with a pretty face, causing independence to give the ticket a second look. Biden on the other hand may help Obama with Catholics and Pennsylvanians. Yet Biden has more foreign policy experience in his little finger than the McCain/Palin ticket has period.
In short Palin is a candidate, Biden is a Vice President. Let's hope the election proves this to be true with an Obama/Biden win.