It seems that there are two qualities of this product. I would investigate both.
I'd pay special attention to that "unstable image" or "wobbly image" comment that is made somewhere in the explanation. Being able to move your eye away from the work, and to then be able to return to your work seems of paramount importance to me.
I have not used one. Someone who has used one should have some much better opinions.
I know that I have used a projector, and I wouldn't think that anyone hearing my comments about working with the typical projector would be tempted to buy one.