No clear "winner".

Obama was better on style and demeanor.

McCain was better, especially on the second half, on "points", but Obama was toe-to-toe with him most of the way, and refused to let McCain’s spun "facts" go unanswered.  In a debate largely about foreign policy — McCain’s bailiwick — that might actually be a "win" for Obama.  In fact, McCain’s taunts that Obama "didn’t understand" or was "naive" seemed to fall flat.

Some early polling of undecidedsfrom CBS News:

40% of uncommitted voters who watched the debate tonight thought Barack Obama was the winner. 22% thought John McCain won. 38% saw it as a draw.

68% of these voters think Obama would make the right decision about the economy. 41% think McCain would.

49% of these voters think Obama would make the right decisions about Iraq. 55% think McCain would.

UPDATE:  Ezra Klein writes:

The first poll I saw tonight was CBS’s analysis of 500 undecided voters. In that poll, 40 percent gave the debate to Obama, 22 percent gave the debate to McCain. Then Frank Luntz’s focus group gave it to Obama. Then the GQR focus group gave it to Obama. Then CNN’s poll gave it to Obama. I haven’t seen any poll or focus group that scored it for McCain. So Obama won.