Good time waster but way too easy. Just research flak first on level 4, then use that to defend until you lift the fog. After that, just defend until you can get automated missile launches.
Just use your pistol until the end of level 12 and buy the laser. It doesn't even get difficult until lvl 16 where it forces you to choose between Zombies piloting airplanes into your house or the immense horde walking. So all in all it fakes a hard difficulty by forcing an immense number upon you that cannot be coped with.
Almost all the Zombies are extremely fast (which is pretty much not a zombie game, there's creatures that look like Aliens and not Zombies, Health packs litter the field that absorb your bullets.
One of the easiest hard badges for infinite mode. Just hit Options, go back to main menu, and select infinite mode again. Viola, brand new level if you couldn't beat your last.