Terrible.
I liked the game up until level 4.
You SEVERELY unleveled the playing field. They shouldn't be able to have zombies that kill 4-5 of mine at once, and have a lot of health. Their also shouldn't be a zombie that's faster than Muhammad Ali and is stronger than most of your units.
I would've rated the game a solid 10 had the last level not been so unleveled. Their's a difference between a challenge and unfairness.