When the framerate of the game is low, especially if it's taken up by logic processing, the new mouse look actuator behaves very 'jittery'.
To reproduce, use Blender 2.73 and run the sample file. Tweak the ITERATION value in the text editor until framerate falls below 60. Notice mouse-look becomes very unreliable.
The same issue was noticed by another user on SE: http://blender.stackexchange.com/questions/25920/