Two player games are hard to test how fun it actually is so I avoid commenting on that to much.
The acceleration on the movement feels a little sluggish, it make precise positioning feel more challenging than it actually is. I like the different bullet types, but they could use a bit more mechanical distinction. The one bullet I don't like is the one that explodes is you're face, as there isn't any planning or choices or interesting decisions made because of it. I would like to see more interesting level layouts, preferably made with special bullets in mind.