In a word...ugh. I don't like the pick.
Sure, distinguished senator, foreign policy credentials, yadda yadda. But there are several things that get me here.
1. The big one: Biden can not help but be a Washington insider. This really, really hurts Obama's message about change. Maybe the campaign is thinking that slogan has taken them as far as it can, and they need to switch tactics to keep things open. Regardless, I don't think they'll be able to use it very well anymore.
2. Biden has a bad habit of sticking his foot in his mouth. This is a real problem on a Democratic ticket, where Fox news is always gleeful to endlessly repeat their slightest mistakes to the public.
3. Biden is also a former candidate. Which means that he has made public statements against Obama. The McCain camp has already started dusting these off and using them.
4. The implications of his experience and geographical position. It makes it pretty clearly that Obama is on the defensive, and somebody in the other camp will probably figure it out, if they haven't already. I expect a strong republican offensive coming out of the conventions.
Of course, it is hard to say how much the VP candidate is going to influence in this election. Hell, both of the Bushes got into office dragging some of the worst running mates ever. Also hard to say how much a vp candidate can help you, unless his name is LBJ. So I could easily be overstating things. Still, I would have prefered Tim Kaine.