Here's a good summation I found on electoral-vote.com --
After months of speculation, Barack Obama has chosen his running mate: Sen. Joe Biden (D-DE). The NY Times, Washington Post, and LA Times all have stories. A text message was sent to Obama supporters at 3 A.M. reading: "Barack has chosen Senator Joe Biden to be our VP nominee. Watch the first Obama-Biden rally live at 3pm ET on www.barackobama.com Spread the word!" Note the fact that he told his supporters to watch him on the Internet. Broadcast TV? Not for the hip.
Biden has an unusual biography as well as pros and cons. A brief rundown follows. He was born in Scranton, PA, to a working-class Irish Catholic family. They moved to Delaware whe he was 10. His father was a car salesman. He went to college in Delaware and then got a law degree from Syracuse University. Lawyers are drawn to politics like bees are to honey and at 27 was elected to the local county council. At 29 he ran for the Senate and won, despite being constitutionally ineligible. Fortunately, he hit 30 before being inaugurated so they let him him. He has been in the Senate ever since and is now in his sixth term and running for reelection this year.
A few months after his election to the Senate, his wife and baby daughter were killed in a car accident and his two young sons were almost killed. He wanted to resign from the Senate to care for his boys but the Democrats convinced him to stay on. He decided to commute an hour and a half every day from his home near Wilmington, DE, to the Senate to maximize his time with them. Thirty-five years later, he is still doing it. In 1977, he married his current wife, Jill. He started an abortive run for President in 1987 but dropped out after he was caught plagiarizing part of a speech from a British politician, Neil Kinnock. In February 1988 he suffered a brain aneurysm but recovered fully.
Biden is currently chairman of the Senate Foreign Relations Committee, a job he has held before. He has also chaired the Senate Judiciary committee.
Like any politician, he has pros and cons. First the pros. Every serious observer of the Senate, on both sides of the aisle, readily admits that few, if any, senators know as much about foreign policy as Biden. He shores up a critical weakness in Obama's CV--lack of foreign affairs experience. When McCain attacks Obama for lack of experience, Biden is going to say: "John, Barack and I are a team and I've been in the Senate 14 years longer than you and I'm chairman of the Foreign Relations Committee, a job you've never held." This alone is worth the price of admission. It makes it hard for the Republicans to attack the Democrats on foreign policy when the Democratic ticket has more experience than the Republican ticket (short of McCain picking Colin Powell as Veep).
From an electoral point of view, Biden also has virtues. While Obama would carry Delaware even with Dick Cheney on the ticket, Biden's working-class roots in northern Appalachia are going to be a big plus during the campaign. He is going to campaign in Pennsylvania, Virginia, West Virginia, Ohio, and North Carolina extensively. He will also appeal to ethnic Catholics in the rust belt. He certainly brings gravitas to the ticket. He is one of the most senior people in Washington and about as prepared to be President as one can be.
He also has some cons. As mentioned above, he plagiarized part of a speech in 1988. The Republicans will hammer him on for being dishonest. He will apologize and say it was a long time ago. Then the Republicans have a tough decision to make. They can say: "There is no statute of limitations on people who are fundamentally dishonest so we can keep talking about this." But that runs the grave risk of the Democrats saying: "Fair enough. Let's also talk about McCain taking bribes from convicted felon Charles Keating also in the 1980s." Then the Republicans can't dismiss that by saying: "It was long ago." If both transgressions are big news, there is a chance people may see what McCain did as more serious than what Biden did.
A second Biden problem is that he has a severe case of verbal diarrhea. He just can't shut up. That doesn't always add to his allure. On the other hand, having run for the Senate seven times, he knows the basics of campaigning and dealing with the media.
A third liability is that he is old (65). It will be tough for Obama to hit McCain for being too old (71) when his own VP is almost as old. Age might have been a strong argument, but with Biden on the ticket, the Democrats can't use that.
Finally, in 2016, Biden will be 73, probably too old to run for President himself so there might be a big primary battle.
Other than plagiarizing one speech, Biden is clean as a hounds' tooth. His first wife was tragically killed, then he remarried. He has never been mentioned in any kind of sex scandal, financial scandal, or anything else. If there had been any dirt on him, it would have come out during his many campaigns.