That's what I'm worried about too. Staring at a screen at a very short, fixed distance for long periods of time can seriously mess up your eyes. I'm hoping that they got good optical magnifying lens inside to help alleviate that problem.
It's much closer than I would hold any device I'm viewing -- a book, a phone, whatever. A comfortable viewing distance would be about a foot away. It seems like it would be a strain, for me to focus on an image two inches from my eyes, over a period of time. I'm sure they've thought of all this, but it does concern me.