First point worth making: it is more fun than it looks. Watching a stream is not the same.
The zombie AI is what you'd expect from a real zombie. They are aggressive, but can be tricked by throwing a bottle, flare, etc. I like that if they know you're in a room, you can sneak out a different way, come up behind them while they're banging on it, and kill them from behind. Once a few of them are chasing you, you're pretty screwed. 1-3 isn't too bad, in terms of fighting or escaping. Once 4+ start chasing you, the others that see them join the herd. Standing and fighting will be impossible with melee weapons at this point, and firing guns at them will lure in more of them. You can NOT play this game guns blazing. Fortunately, the stealth mechanics work well enough to keep it fun.
So what you're saying is this video isn't accurate: http://www.youtube.com/watch?v=rC-HBQbeLgU
Because that goes against everything you said about the AI.