As has been said, the controls are very good. I appreciated the variety of enemies, and the upgrades managed to keep things interesting. The music really held it all together.
My only real complaint is the ammo. This game is VERY stingy with its ammo, and the first several levels have to essentially be played with a melee weapon. Certain enemies, such as the black soldiers, eat up your ammo, so much so that killing a group of them will eat up half of your total ammo, even on the later levels. Extra ammo can only be gotten from objects lying scarcely around the level.
Usually when a game has low ammo, it has powerful melee weapons to compensate. While melee upgrades are available, there are only three of them, while there are no fewer than fifteen guns. None of them have sufficient ammo, the highest ammo count any one of them can have WITH UPGRADES is about 170, give or take 10. There are 3 weapon slots, but going in with 3 guns is suicide, except on far later levels. This problem is compounded since even the strongest melee weapon still takes multiple hits for some common enemies, and often doesn't hit more than one or two enemies when slashing at a group.
To improve the game, I would suggest one of three things. One, have random enemies drop ammo. Two, have more available melee weapons with upgrades. Three, fewer high health-ed enemies like the green and black robots. I could only progress through the game without dying by avoiding them, which makes it too heavy on dodging, and turns it into more of a chore than a shooter still be. Despite all my criticism, it killed a few hours, and I genuinely did enjoy it. It can improve, but it is already better than many online games.