The X axis mouse speed is always the same (looking left and right). However the Y axis is not (looking up and down). It tends to accelerate and makes things very akward. Not sure if by design or an actual bug, regardless mouse speed should be consistant in Y axis cause it makes no sense for it not to be.
Please note that this is with:
Mouse smoothing = off / 0
Aiming deadzone = off / 0
Mouse acceleration = on / 1 (see issue 0002415)