The title is 'World War Z,' but I can think up two other letters: "O" and "K."
'World War Z' is okay because it zips along with the fury of a computer-generated cascade of fast zombies. 'World War Z' is okay because Brad Pitt is a great leading man, even if his character has no depth. 'World War Z' is okay because there is always a fatalistic draw to see our social order tumble and great cities reduced to cinders.
It is also, unfortunately, merely okay because there's nothing in this movie you haven't seen before.