Just bought this today...£6.99 on steam. Lots of fun

The missions are all quite similar. Kill zombies in an area to rescue the survivors and take them back to your base.
Good game to kill a little bit of time as missions are only about 5-10 minutes long.
If I had to sum up the game in a sentence it would probably be:
"Arcade-style driving from the original GTA games with zombies + guns"