Great game.
I feel a bit bad having to give this a 9 instead of a 10, but it's not like the game was perfect.
Let's talk about the gameplay: Kill zombies. Yay! That's fun. The four different modes didn't sit well since they were basically all just "kill." Don't Touch My Gems was better since it had more of an objective to it.
Achievements: A good idea that added something to aim for, like all achievements should. Good balance of difficulty with them too.
Weapons: Very fun to use a large variety of weapons, but some just plain sucked, which is reasonable because some of the bad weapons were gotten for achievements that were easy or for you sucking (missing 400 times). Some of the weapons felt too similar (namely the rocket launchers. They felt like the exact same thing. Maybe they had different blast radii or different damage; my bad if I didn't realize it).
Controls: Switching between weapons with a simple left and right was not a good idea. Perhaps adding keyboard shortcuts would have been good. I always found myself just going to the inventory to switch weapons. The screen panning wasn't all too great, which I will talk about next.
Design: Things felt a bit too small, especially the panning screen. I think it would have been fine to have just shown the entire area on-screen.
Having the achievements and buttons at the bottom overlap the area felt too obstructive and in the way.
But those were all minor problems that didn't affect the gameplay too bad. Loved playing this.