I will start saying that Im not used to this kind of shooters, I mostly played TF2 and doom 2, so I might be just bad with quake 3 kind of games.
I first tried out the game in the third difficulty thinking it was the default one and got absolutely destroyed. The quick and small enemies, in addition to my high speed made everything too hard to hit. Also I ran out of bullets in a flash. The floating green balloons were specially difficult, since they dropped my speed harshly and came from nowhere since I was mostly watching at the floor from jumping.
Once I swallowed my pride and choose the easiest difficulty, I liked the game a lot more. I still ran out of bullets too quickly though and the difficulty went through the roof with 6+ floating octopuses after the initial waves, which was absurd.
The AI of the runner enemies seems to get confused sometimes, they go straight at you and then sometimes run to a random place once they are too close.
I really don't like that damage slows you down so strongly. It feels like playing against a natasha heavy 24/7. I couldnt find a use for slowmo button. Weapon select could really use the mouse wheel to cycle through the weapons faster.
All that said, I enjoyed playing in the easiest difficulty. A lot of weapons felt powerful and the melee monsters are fun to shoot (the green balloon can go to hell though, really). With a proper way of getting more ammo and a good difficulty curve, I can see myself trying to get a high score.