Ezra Klien has a good point about the split between the voter reaction and the pundit reaction to the debate. McCain played to the pundits and won over the pundits. Obama played to the voters and won the voters.
Not everyone who watches the campaign is equal. If you would indulge me in a sports metaphor for a moment some voters are fans and some are the ball. The fans know who they are voting for. They are cheering for their guy to do well and the other guy to stumble. The undecided voters are the ball. They are who the candidates are playing for. The guy who gets his fans cheering loudest feels good about himself. They guy who wins the ball, wins. The pundits in this metaphor are the color guys in the announcer booth. You can impress the hell out of John Madden with your gutsy play, but if you don't put points on the board. Likewise, David Gergen could rule you the winner all day, but if the voter don't swing your way, you have lost my friend.