I finally got around to seeing this last night, and I wasn't disappointed. The first half is straight-up one of the better zombie apocalypse films ever made, set on a bullet train from Seoul to Busan. The pacing is tight, the characters are interesting and flawed and downright twisted in meaningful ways, and the chemistry between the two main characters is properly antagonistic.
Things do drag a bit in the second half (the whole thing feels like it could have lost 15-20 minutes) but the ending is strong.
Two things that stood out:
a) When the zombies come to South Korea, take comfort in the fact that you will be bitten in the neck by a creature with perfectly white, perfectly shaped teeth.
b) No guns. I can't think of any other zombie flick that exists where guns don't play a major role. Refreshing, really, and very Korean. Real men and women will use baseball bats when the apocalypse comes.