I always find games where the bullets are not significantly faster than the player's movements to be very interesting, because you get some kind of "suspended animation" effect as you expect the bullets to zip by. It also subconsciously changed your shooting patterns a bit. It also allows you to dodge enemy bullets which is nice.
I like that the game forces you to be proactive in shooting the spawners. I think overall it's an effective game for what it tries to accomplish, although I think right now the rooms are a bit too repetitive. One option would be to have different enemy types with different behaviors, another option would be to vary the rooms shapes a bit (right now they're all rectangular pretty much).
I noticed a couple of bugs: When I switch the particles to GPU mode, the particles don't get rendered anymore. Although I actually preferred this as it allowed me to see the bullets more clearly. Also I'm not sure how the leaderboard works right now. The top place was at 69 points and I got 95 and didn't get on the board.