Now that interest has renewed in the demo due to the current Kickstarter, I've been receiving reports of mouse look issues for some players.

It would seem to be a framerate related bug. If you find yourself having issues with mouse look (sluggish, inconsistent, acceleration, stuttery while the rest of the game is smooth, etc...) try turning the FPS to 60 in the Options menu. So far all reported mouse look issues seem to have been solved by this. Sorry for the inconvenience!