I wouldn't dismiss her as a bimbo quite so quickly, for a couple of reasons. One is that that kind of attack is a fairly classically sexist way of dismissing women, and it's one that tends to backfire. The worst things the Democrats could do is ridicule or underestimate her - that would play into McCain's hands. Biden will have to be especially careful in the debate not to appear to talk down to her in any way. What I gather is that she's smart, well spoken, and apparently honest.
That said, I think it's a cynical choice - a bald-faced play to get the disaffected Hillary voters to vote against their own interests. Some probably will, but I doubt that many - especially when her record on choice is made clear. She is young enough to make McCain look very old by comparison, and as others have said, her resume makes Obama look like an elder statesman by comparison.
It will certainly keep things interesting.