We played with 12 Zs against 4 characters - not quite balanced, but it was a test run.
Zombies are quite scary when they move in mobs - as soon as the mobs start to fall apart, the survivors gain upper hand - so the trick is to have a table with not too much open space for the survivors to move in. Fear tests work like a dream.
EDIT: They are also very hard to kill
I'm really impressed with the way the 'Song..' engine can be used for so many different settings by adding a few twists and bits and bobs..