I wouldn't underestimate her. She gives a great speech. She is likable, and she has charisma. On the video I saw of her in previous debates, she held her own, so who knows.
I feel she'll have some good one liners and zingers to make her points. The uneducated voter may like that and feel she's "winning". Beyond that If she can keep her answers short and they don't follow up or push for details she may do better than everyone thinks. I think alot depends on Gwen and how hard she pushes for details. In the past, that is when Palin got lost in her talking points. I agree Biden needs to say less and let her talk more to show her weakness.