For their low points cost it has to be said Zombies are rather proficient in Combat (unlike Skeletons), but this is of course offset by their high Quality (I know that doesn't sound right) and the Slow and Short Move special rules. It's generally quite simple for most opponents to dance around them.
Zombies has to be a numbers game.
Echoing everyone else: if you're going to take Zombies, takes loads of them!
A Bailey Bridge made of bones would be such an awesome modelling project for an undead army!
That would be very sweet!