Those who don't suffer from motion sickness and enjoy the realism it provides could keep it on full blast, and those that do could tone it down or turn it off completely.
It's not even realistic how the character rocks his head back and forth like that though..
Personally it does not make me sick but I did notice it was quite intense and when I played a little Legacy I had a great feeling of stability and focus. I very much like games where the aim is extra precise and the view very raw, static, clear (not blurred by motion blur etc). I think the best examples of this are the Counter-strike series.