My friend had the DK for the Oculus, and his friends bailed unfortunately on developing for it.
That said, I used it in a cool demo created for it, a slow First Person exploration game that was extremely creepy, and moded Half-Life 2. I was super sick during Half-Life2, but the factor is that game was not created for it. No one can move their head then slam a mouse around to turn around in real life. The FP creepy game gave me sickness too, but not as bad. And it was just the act of walking forward with a controller, but my head controlled the looking.
That said, I am prone to motion sickness. But it was worth it IMO. The best application was the demo where I just looked around, and as I did, the world changed. It had a awesome ending; it is unexplainable how it was so immersive, so refreshing, and so different. It's like describing a tube amp's tone vs. older digital amps. We start to use words like warm, saturated and so on that don't describe the true nature of the sound. Same goes for VR- it's just so much more amazing then starting at a screen with a controller.
Overuse of VR - out of touch with reality- is a real risk. But the potential is incredible, and I actually have been waiting for the next frontier in video games. IMO, it's been the same for the last few cycles; new graphics, same experience. The Wii at least tried something new; the VR takes that even further. Someone can go to a VR foreign county, and get a great experience if they are unable to go.