An interesting novelty, like the Laser keyboard only worse. There is no layout to reference. Not everyone can touch type. And for those who can, there's not tactile reference either, so... yuck.
If you type on the surface of the desk, it's going to hurt after a while. Even more if you type on "air". The effort of keeping your hands in the air while typing is a lot more than being able to type on a physical board.
The only use case I can think of is "typing" on your legs (since you can then allow your arms to rest, have "tented" hands and a good non-solid tactile surface for typing on). With an on-screen reference for each finger, showing their positions relative to the "keys". Could work okay, then.
Then again, if you're typing with both hands, you're not likely to be using a portable device such as a tablet or phone.... so just:
Get a mechanical keyboard. Accept no substitute.