Good for 5.minutes each time you open it and that's about it. Gets monotonous and boring real quick.
No enemy variations past level 15 and the scaling is really awful. Power ups are rare and it melts down to how much you can upgrade your guns and level up to unlock the next gun until you get a rocket launcher at level 60 and that's about it.
Power ups have some cool things like an increased fire rate or angle but they don't drop often and they don't last long either . There's a shield that drops often but it's a one hit only, and a car which becomes useless pretty quick because it doesn't scale up in damage so it becomes a hindrance to use.
No matter how much you upgrade the guns zombies are still stronger, there's no real value in upgrading other than damage numbers which you need to keep up with the hoard.
Character perks can be upgraded too but they're expensive and don't add much to the game play. You can do without them entirely except maybe health because it gives an extra heart.
Pros:
No ads.
You get free game currency (MDT) every day + gems
You can purchase gold and revive for 1 MDT which is a good value
Nice blood effects
Cons:
Gets boring real quick
No real value in upgrades
Power ups balance and scaling is bad
No enemy variety
No levels variety
No bosses variety (only the same 3 bosses)
No gun variety and no satisfaction in upgrading them or damage values
Final rating
Good if you're bored to tears from everything else. Otherwise kinda ehhhhhh