For the record, I'm very happy with the Biden pick. With the possible exception of Al Gore or Jim Webb, I can't think of someone who generates as much excitement while also boosting Obama's electoral prospects and his ability to, you know, govern. (Hillary might arguably belong in that group, too.)
So far as I can tell, Biden also brings three significant liabilities, but I think they'll turn out to be far less damaging than you'd expect. Let's go through them individually:
1.) The plagiarism flap: No question that if you have a choice between two nearly identical running mates, only one of whom has plagiarized, you opt for the other. Still, I think it's going to be hard for the GOP to exploit this, and pretty easy to overplay it if they try. I'd guess voters will find it pretty unfair to indict a guy over a minor 20-year-old slip-up which he's repeatedly acknowledged and abjectly apologized for. More importantly, as my wife (Time correspondent Amy Sullivan) argued this morning, something tells you the McCain campaign doesn't want a fight over 20-year-old scandals. (In fact, you could argue that McCain himself is proof of the first point.)
2.) The gaffes: Again, all things being equal, you'd prefer a guy who wasn't as prone to rhetorical unforced errors. Having said that, I think Biden's gaffes tend to be pretty superficial. They don't reflect a fundamental lack of knowledge or empathy--far from it--so much as a certain ham-fistedness in putting words together. This is almost entirely forgivable and unlikely to create long-term problems.
More importantly, to the extent Biden has sometimes gotten tangled up on the subject of race (see here, here, and here), I think it actually helps Obama. There are a lot of voters out there still getting their heads around the idea of an African American nominee. Among other things, they may not yet have the vocabulary to discuss Obama's candidacy without expressing some of their thoughts inartfully, and they don't want to get pilloried if they trip up. The Biden pick tells them, in effect, don't worry about it, this is tricky for all of us. It reassures them that an Obama presidency doesn't mean every word they speak will be parsed by the sensitivity police. And, perhaps most importantly, that Obama himself isn't fazed by an awkwardly-worded sentiment. This strikes me as pretty critical.*
3.) Previous Biden statements about Obama: Sure, in a perfect world, the veep pick would have spent the primary season slavishly praising the wisdom of the eventual nominee. That obviously didn't happen in this case, and the McCain campaign has wasted zero time pointing it out. (See this insta-ad, for example.)
On the other hand, I think this is another one of those seeming liaibities that could turn into an asset. That McCain commercial has Biden saying Obama isn't yet ready to be president, but could be. That comes from a debate that took place over a year ago. A lot has happened since then. Is it so hard to imagine Biden would have seen enough from his ticket-mate to be sold by this point? Doesn't this in some sense make him a more credible witness? I mean, I doubt Colin Powell or Chuck Hagel thought Obama was ready to be president when he first started running. But if they came out and endorsed him now, it would be a powerful statement. (Obviously Biden's in a different position since it's now his job to sing Obama's praises. But you get the point...)
For that matter, I'll be a lot of Democrats have made this same trip, from skeptical to won-over. It will hardly come as a surprise that a Senate veteran has, too.
*I can't remember for sure, but I think I've chewed this point over with reader RY--you know who you are. (Possibly in the context of Jim Webb.) If so, I owe you some credit here.
Update: Commenter Brent mentions a fourth liability I meant to address but then forgot about. I think his take is exactly right:
[T]here is a fourth downside: that Joe Biden has said very nice things about McCain in the past. However, the jujitsu on that one is very easy. He said those nice things about John McCain before he turned into the pathetic, craven, neo-con McBush that was able to get the Republican nomination in 2008. Done and done.
Agreed. It actually plays into a narrative Obama may want to establish, which is that McCain was a pretty reasonable guy until his ambitions won out.
Second update: Here's an interview I did with Biden in early November. He talks a lot about his foreign policy experience relative to the Democratic field.