Biden is serious, smart, and will take the debate to the Republicans. No one has more gravitas on foreign policy than Biden. He could step in as President if need be (God forbid) and do a fine job. Yes, he's loquacious, and yes, he'll put his foot in his mouth every so often. But if those are his worst flaws, well, I think we can live with that. This is a great ticket for the Democrats.
I only wish Obama had not buried the choice in the recesses of the news cycle -- late Friday night/Saturday morning? And on the eve of the Democratic convention? Why not do it earlier and hope to get two bumps rather than fold the two major events into one bump in the polls?
But in the end, the choice for VP is a test of judgment. Obama has passed that test. Obama/Biden. This is my favorite ticket in my lifetime, without a doubt, and is historically pretty damned sound, at least from the perspective of less than twelve hours.