Not Bad, just a few things though
1) Control the aiming with the mouse. Having only 4 directions to shoot gets in the way
2)'Optional' 2 Player mode(Though aiming with mouse would get tricky.)
3)More guns. Need I say more.
4) Maybe a bigger arena. This one was fine, but with 2 players...might want to consider more room.
And like the guy before me said...being able to customize your own controls