I thought it was a solid performance by both candidates. Neither candidate blew the other out of the water, but neither killed his own candidacy. If anything, I would say that Kerry may have refuted the idea that he would be weak internationally, at least to some minds. Bush, on the other hand, did a pretty good job of injecting the names of leaders and policy specifics to counter longstanding charges that he is a lightweight.
Of course, it depends as much on what the media says for the next few days as it does on what the candidates said in the debate. It should be interesting to see how the spin spins.
UPDATE: Here's Giuliani spinning on the Daily Show. Bush spoke, Kerry lectured. Kerry flip-flopped in the debate when he said the war was a mistake, but then that Saddam was a threat. Except that Kerry said that the manner in which the war was initiated and conducted was a mistake, or more broadly, the way that the President chose to deal with that threat was a mistake. I don't see that as a contradictory position. 1. Saddam was a threat. 2. There is a good way to deal with a threat like Saddam, and some bad ways. 3. The president chose one of the bad ways.
Not a flip-flop, in my mind.
UPDATE 2: Matt Yglesias at TAPPED on Bush inaccuracies.