As it happens:
My first thoughts after the first two questions:
Kerry sounds more rehearsed--I don't know if that's good or bad. His initial answer was a bunch of general statements. "I have a plan," etc. That's a problem for him... Oh, question three--he just brought up Vietnam. Didn't take long.
Bush stumbled initially, but picked it up near the end of the first question and has been pretty solid since then. His answers are more specific, and at the same time, seem more heartfelt.
First round: Bush
UPDATE: Kerry hammered the "why hasn't he gone after bin Laden?" issue. Bush: "Anyone who thinks that bin Laden is the one issue in the war on terror doesn't understand the war on terror." Ouch. That one drew blood.
Seems like it's getting a little ugly...and repetitive. I'm not sure there's any clear leader here, although I don't think Kerry's getting any points in the national security department, as he seems to be doing nothing but criticizing those currently involved without ANY indication of what he would do to make it better.
UPDATE 2: Bush is getting to sound a little repetitive. That's hurting him. As Ken said in the comments, he's wearing out the "wrong war, wrong time" defense, and now he's wearing out the "my opponent flip-flops, and you can't lead if you don't take a stance" bit.
UPDATE 3: Tim Russert just said that Kerry "found the voice of the Democratic party." I didn't sense that at all. In fact, I think that was Kerry's biggest failing here. He had a perfect opportunity to enumerate the policies he would put into place, but instead he stuck by the Nixon-like "I've got a secret plan; elect me and you'll find out what it is." I know absolutely NOTHING more about his ideas than I did before this started. I don't think that Bush was all that enlightening (except maybe for the North Korea situation) either, though.