I would give Obama an A, his performance was good, his arguements were convicing even if you dont agree with them. Althoug he din't always answer the question, he did however answer more questions with a yes or a no than any other candidate.
Here is what TIME had to say on the subject.
When he was good, he was very, very good. When he wasn't, he was dangerously close to ordinary. Spoke movingly — even/almost presidentially — about America's troubled history with race, his favorite teacher and how hard he would fight for universal health care. (Obama even drew an approving nod from Elizabeth Edwards in the audience on that last one). Took occasional soft shots at Clinton on Iraq and her questionable ability to fight for change (his main theme), but never made a decisive impact. He began many more sentences with, "When I am president" (without the conditional) than he has in past debates, but his own video (all the candidates made them, and they were sprinkled throughout the telecast) hit more Kennedy chords than the live Obama did. Still, his best performance to date, positioning him to return in later forums to the change-change-change contrast he wants (and needs) to define his candidacy.
—by Mark Halperin "
( Time )