I attended my brother's law school graduation ceremony at the University of Minnesota in the late 80s. Joseph Biden was the commencement speaker. My recollections were that he had a good speaking voice but didn't anything noteworthy or compelling, e.g. where's the beef? His speech was a political one; he officially announced he was running for president. Also, others in attendance reminded me he didn't mention the graduates or give them any advise. A while later, he was found to have plagiarized material for a political speech and during law school and was forced to drop out of the race.
A problematic aspect of adding Biden to the ticket is he's already made the case that Obama is not ready to be president; he lacks the necessary experience. He said so much during a presidential debate. Not surprisingly the McCain campaign already has a campaign ad highly Biden's comments about Obama made in Obama's presence. The question is what's changed?
Again, ultimately the vice presidential candidate won't ultimately decide whether voters will vote for the presidential candidate unless he serves as a drag on the ticket by unnecessary faux pas.