I would have to be concerned about your vision after using the device for prolonged periods. Even after using the regular screen for a few days, when I go into the city and I have to increase my range of sight, because not everything is right infront of me, and I can sometimes get a strange sensation of discomfort. My point being that since this screen completely covers your line of sight, and since the picture on the website shows the guy sitting right in front of the screen, I'm afraid that it could inflict a worse case of the discomfort I mentioned.
This is similar to how people say if you sit too close to the TV your eyes will go wierd. Technically that only happens if you try to focus on the screen really hard (like read small writing, which you might feel inclined to do if you're sitting too close). But if you had the same case with a computer screen you will have to read that small writing, and I fear people may get the same repercussions for misusing this device.
I suggest sitting 1 metre from the screen atleast. Please heed my warning.