I tested being a rear gunner in a Bf-110 flown by a squad mate last night on the Repka #1 server.
It cannot be so, that when I drag the mouse whole length of my mouse pad the gun turns just about 5 degrees or so. About 10 drags to get it from side to another. So, we are flying near England and surprise surprise, we get a Spit on our six

.. The gun is useless, as when I start to drag in the direction of the Spit, the Spit moves, and there is no way I can keep up. Only when the Spit makes the mistake of parking on the six I get a shot or two off.
The firing button is buggy too. I set the firing of the gun to right mouse button for obvious reasons. When ever I pressed the button, only couple of rounds were fired even when I kept pressing it. So I had to go mad on the button to keep firing.
On top of this, the firing sound stuck on, so the rest of the flight I was emptying the magazine

.. Quite annoying.
One positive thing was the kick of the weapon. As I fired, the gun bounced a bit so I had to adjust it (the slow movement didn't help at all

).