Its a fun little game, but the "freemium" nature of it has been jacked to the max. Its very difficult to play enjoyably without forking out some dough, most notably because of the Energy system. It's similar to ammo in Metal Slug Defense, but as far as I can tell there's no way to upgrade it, and you chew through it very fast, especially once you get to more difficult levels.
I don't mind tossing SNKP $10 or so, but that won't get you very far. You have to spend big bucks to really get out of the game what it has to offer.
What's worse, and this is one of SNKP's biggest flaws for the mobile offerings, is all save data is saved locally on your phone. If your phone is bricked, broken, or lost; you transfer from one OS to another, or transfer to any new phone without knowing how to migrate your settings, you're screwed and forced to start over from scratch.
I'm honestly hoping that a working hack tool for the game comes out just so I can get around all of the paywalls.