My problem with running zombies...
... is that it's just impossible to escape! I mean, it's somewhat like a land-race between Usain Bolt and a goldfish. They have zero margin for error. One mistake, took one extra second to get to the door and you're done. So they have to try not to kill one another (as most human beings who are forced to hang around other people whom they don't particulary like for prolonged periods of time), keep running their asses off and have lots of ammo, avoid corpses that fight with bestial fury and run like cheetahs on speed...
Their chances are null. Sure, it's fast-paced and exiting, but it's somewhat boring, because you KNOW they're going to die. Everyone. There's no way at all that it's possible to survive.