The mutation mechanic is excellent! I love the way it impacts your stats, and encourages you to seek out specific enemies to get the stats you want. I also like how it forces you to get close, and the zoom in+sfx make it feel great to use. There's not much variety to the enemy attacks(projectiles aimed directly at the player), but the cramped level design+speed of some of the bullets keeps you from simply circling around the enemies until you win. The enemy spawns being spread out helps too, and it feels like a lot of thought was put into the AI and how the enemies move around.