Way too hard, and being forced to split your energy between different modes on the same gun is stupid. You could easily rebalance the game to only use one upgrade screen, and it would be better for it.
WHY DO I HAVE TO PRESS A BUTTON TO RELOAD? It's 2009, science has brought us the ability to make our videogame characters reload on their own.
And why do the zombies have 10-foot-long invisible arms?
Why do I have to pick up blue dots in order to get money for my kills? It's a tad counterintuitive to ask the player to fly into clouds of enemies just to pick up the reward they already earned, especially when it fades away for no better reason than because stuff tends to fade away in games. I thought we'd grown beyond that kind of inconvenience.
And the controls are way too sensitive.
 "Red Code 2"){kind=icon}
 "Wrath of the Empire"){kind=icon}
 "Zompocalypse"){kind=icon}
 "Heavy Weapons"){kind=icon}