a) Is it really, truly, useful?
b) Can it become cheap enough for real world use?
I doubt both, right now. Displaying movies "somewhere in the air" is not really useful. A video beamer can use any patch of white wall today - or you can simply use a display, which is probably better for the picture, anyway... Computer displays... I dunno. Fun, maybe, but truly useful? Doubt it.
When it matures enough to be viable from a cost standpoint, it'll be revolutionary. That's probably quite a few years away though, and not every "use" will work well with such a technology. Doing presentations at a museum or trade show might be a strong point, but designing with such a display probably won't be too easy with this technology.
pretty cool, if the "projector" for these things takes up less space than an LCD does now adays, it's plenty practical. But i have a feeling that currently a device like this draws a lot of electricity. as far as using them as 3d displays, they're going to have to wait for applications to be written for them, and for the price go to down considerabley to compete with the up and coming "Organic LCD" which will soon replace our flatscreen monitors that we have. Someday... Someday...
all this makes me think of is that scene in the original starwars trilogy "help me obi wan kenobi, help...."