I really like how it is quick to action, and the art/graphic style is pretty impressive!
The game took me sometime to understand though, especially the jump button (never knew it has a cooldown, maybe you could add the 5sec countdown timer text on top of the jump key, with a slight decrease to the jump arrow's opacity to avoid obscuring the text).
Only other things are the slugs (pickups) are a bit small, maybe you could make them slowly flash or increase their size, not a problem after you played the game a couple times, but took me a while to find out that they basically lie there for me to retreive them after a shot.
The jump is pretty fun to use, especially when dodging projectiles, and I like how you gotta retrieve slugs, which means you gotta manage ammo (slugs) and punishes spaming, adding extra depth to the game. Great that they surround you to make it more challenging. You could improve it by adding control key icons (with a white text to stand out) on the Slug icons, bottom right corner (this way you know which keys are needed to fire each slug, making combos easier).
Hope this helps and was detailed!