I enjoyed the variety of weapons, particularly the revolver - the one shot kills but short time between reloads is a nice trade off given the small size of the arena. The difficulty curve is good, and I thought that the forced weapon switching worked well for the limitation.
As with Dundy3 below I didn't get any audio from the 3rd wave on, and would agree with other comments that a little more visual feedback on bullets and damage would have been great.