Jay Cost over at the HorseRaceBlog gives his thoughts on the Biden choice:

Of course, Biden has plenty of upsides. He proved a good debater during the primaries. He has lots of experience, and so he might provide "gravitas." He has a working-class Catholic background, so he might appeal to some of the FDR Democrats who liked Ronald Reagan and Hillary Clinton. Plus, he's tough, so he can go after the Republicans.

On the other hand, his windiness is a big downside. I get the impression that most insiders who know Biden kinda like him - but they probably prefer small doses. I know I feel that way. Biden makes me grin when I watch him for 10 minutes or so. But after two months of Biden every day? I'm going to need a break from old Joe by then. And then there is the non-trivial possibility that he will say something catastrophically stupid.