I really wanted to like this game. The idea is excellent and the game is impressive from a technical standpoint (hundreds of enemies and projectiles on screen!), but it falls short on the execution of basic mechanics of both the FPS and roguelike genres.
On the FPS front, the guns have no kick to them at all. They're all peashooters firing peas of different size, speed and damage - they have no recoil, no knockback, no stagger. Though the weapons seem very different at a glance, they all play pretty much the same - you hold down M1 while circle strafing a screen full of enemies. The gunplay and movement feel floaty and unsatisfying, and there is no feeling of in-game presence, no meaningful feedback from the guns and thus very little satisfaction from the core gameplay.
On the roguelike front, the game lets you choose one gun at the start, and that is the only gun you will get in most playthroughs. After 30~ playthroughs and about 10 hours of gameplay, I have seen another gun exactly twice. Most items are simply passive stat upgrades, and the active items are incredibly rare, take a long time to recharge and are mostly useless. More than once, I've went several levels without an item or shop in the way.
To top it off, the game has a bug (at least on the Linux version) where a single level will run at incredibly low framerate for no apparent reason. As the game is unplayable at 10 fps and there is no "save and quit" function, your only option is to abandon the current playthrough.
I cannot recommend this game in its current state, and its flaws run too deep to be fixed by patches.