UNPLAYABLE.
This has very little to do with "MOUSE ACCELERATION", and as many have already noted, is instead "NEGATIVE MOUSE ACCELERATION". Regardless of the various arguments about what "feel" the developers are trying to achieve, in no game is the crosshair moving slower, the faster you move your mouse, a good thing.
Also, when moving the mouse quickly to the left you find yourself looking up and conversely when moving quickly to the right, looking down. This makes even less sense than the negative acceleration.
I will be monitoring this issue; but for the meantime, I will be uninstalling until there is a development here. Simply unplayable. Who would have thought that BASIC INPUT is a lower priority than COOL HATS.
"RAW INPUT" should be a very high priority at this point.
(Arma 3 already has correctly working mouse input code; just steal that)