A bunch of reaction from around the web. Ambinder:

Obama-Biden will be a formidable ticket, and a risky ticket, and not a comfort zone choice for Obama. "It's a big ball pick, not a small ball pick," an adviser said...

I gather that what impressed Obama about Biden is that Biden gets things done. He's a man of action. He's not a bullshitter.  I also get the sense that Biden, 65, is pretty well aware that, at age 73 in eight years, he's not going to be a viable presidential choice, and thus was able to convince Obama that because the vice presidency would be his terminal position, the famous Biden ego will take a subordinate role.

Patrick Ruffini:

When Bill Clinton chose Al Gore, this was rightly seen as a template for modern VP selections. The traditional notion of "balancing" the ticket with someone of a different state or background gave way to the idea of using the VP as device to reinforce the message of the guy at the top. So, in the last change election, you had two young Southern party modernizers on the ticket. You couldn't argue their message was not about change.

Instead of reinforcing Obama's message, Biden muddles it. Biden is the ultimate Washington insider, having been in the Senate for 36 years.

Jay Nordlinger:

I believe that Barack Obama will not wear well.

Indeed, I think his act has been wearing thin for weeks. And I think that his vice-presidential nominee will wear even worse. I’m not sure why he went with Biden - maybe he likes him, personally, which is not the worst thing in the world. It’s helpful if a president likes his vice president (or at least doesn’t dislike him). But I don’t see what Biden gains him. Biden is a slightly risible figure, what with his hair plugs (or whatever) and his many, many examples of public obnoxiousness. All politicians have sizable egos, but this may be the most self-loving ticket ever.

Fallows on Biden's debate performances and his disappointing Senate questions:

Politicians have to be egomaniacal to be in the business. Anyone who enters the US Senate with a limited appreciation of self soon has it expanded. But while Biden's ineffective hearing "questions" often sounded as if they came from "normal" Senatorial egotism -- I'm on stage now, listen to me -- his debate comments and his partisan anti-Bush arguments reflected a more attractive egotism of knowledge and policy. Let's call it simple confidence, of the sort that Bill Clinton in his prime displayed when dismissing Republican economic arguments. The subliminal message in this pose is: I know what I'm talking about here, I've dealt with this for years, and I have no time for the other side's ignorance.

David Corn:

Biden comes with decades of baggage. There are thousands of Senate floor votes for GOP oppo researchers to sift through. He's had more than one plagiarism scandal. Hailing from a solidly Democratic state, he brings no Electoral College votes with him. But he has the talent to be both Obama's attack dog and his top foreign policy adviser. And though vice presidential nominees tend to have no true impact on the final results, Biden has the potential to be a fierce campaigner for and with Obama--that is, if he can be the better Biden for the next ten weeks.

James Poulos:

There are three main things you want out of a Veep.

(1) The ability to appear in public 24 hours a day, uttering incessant and high-profile attacks on the opposition, without overshadowing the Presidential candidate.

(2) The ability to shape the office of the Vice Presidency, post-Cheney, into something more than useless but less than monstrous.

(3) The ability to square dispositionally with the Presidential candidate without disappearing into his aura or echoing his every instinct.

This is why Cheney was an exceptional (1 and 3) but not perfect (2!) veep, whereas Biden is an extraordinary, almost perfect choice.

Dean Barnett:

I must offer thanks to the Obama campaign – the Biden selection promises to provide grand entertainment. On the right, we’ll get to dust off Joe Biden’s greatest hits, a pleasurable task that will take weeks. Also on the right, we’ll get to watch progressives feign joy over Obama’s elevation of an Iraq war supporter who enjoys a cozy relationship with the credit card companies (not that there’s anything wrong with either one of those things).

Like many people who read Richard Ben Cramer’s seminal “What It Takes,” I’ve long harbored a secret soft spot for Joe Biden. But that soft spot is always threatened by prolonged exposure to the man. Usually a 20 minute segment on Meet the Press provides a mortal threat to my lingering fondness for Biden. Like the head of the Democratic ticket, Joe Biden tends not to wear well, especially in concentrated doses over short periods of time. Speaking purely analytically, Obama has made a poor decision.

Hilzoy:

I think this is a decent choice, especially given the alternatives (Bayh, shudder.) I've seen at least one article -- regrettably, I can't remember which -- saying that it was a safe choice. I don't buy this. Obviously, there were riskier choices Obama could have made -- he could have picked Courtney Love, for instance. But there are obvious risks to picking Biden: the plagiarism from the 1980s, the fact that he is, as everyone says, "gaffe-prone", and so forth. The truly safe choice, for Obama though not for the country, would have been Bayh. I'm glad Obama didn't go that way.

Jonathan Cohn:

I imagine Biden's selection will indeed make it harder to run against Washington per se. And that's ok with me. I've often thought that the Obama campaign had become too focused on changing the culture of politics, rather than improving people's lives. If picking Biden forces them to do more of the latter, that's probably a good thing.