I've heard that everyone is impressed with the lack of motion sickness with Valve & HTC's Vive headset. I think the major difference is the "low-persistence" display, which flashes each frame for a tiny amount of time instead of leaving the screen on all the time. That way the persistence-of-vision is a little more normal and it doesn't smear the image across your eyeball as much. The latest Oculus prototypes have the same feature.