A great idea, and very ambitious. I mucked around for a bit but unfortunately wasn't able to play against anyone. The gravity mechanic is very cool but I admit it is a bit disorientating - I would not make a good space marine. I like the amount of weapons you included.
I did find the platforming a little bit off, I think a bit of ramp up of X velocity would be a bit better than instant full speed. Otherwise though, a solid little multiplayer shooter.