I think this is a horrible choice, for a few reasons:
1) He is extremely unlikeable. I live in the area, and while he has strong supporters here, his attitude and demeanor are ver "north east". Meaning: He can be a snide asshole to make a point, kind of like me.
That doesn't fly well in other areas of the country.
2) Obama is running on a "no experience necessary" ticket, and he chooses Biden who could only have been chosen because of his foreign policy experience, and Obama is going to have a hard time making those two things coincide if he is challenged on them.
3) Biden calling Obama "clean and articulate". Trust me, that WILL be resurfacing.
4) Ardent Hillary supporters will be pissed, and considerable (not majority) amount will vote for McCain
5) Biden is also known for making some rash comments when challenged, and that could and will be used against him. He does not have a "presidential" demeanor... well not in this centruries idea of presidential anyway.