The big gain I see is enabling more controls on smaller devices. Right now physical size is a hard limitation on the number of different control interfaces (buttons or screens or, for future devices with Soli or it's kin on board, gestures), with Soli though the whole volume above the object becomes interface space and the limit is the number of gestures that can be recognized.

I wonder if you could use this for a battery free controller. If you can detect the orientation of thumbsticks on the controller, or whether or not buttons are being depressed, it seems you could have a simple and cheap device for a controller.

The virtual watch crown in combination with Apples haptic engine would be awesome.

The HCI win doesn't come from making gestures in thin-air, it comes from being able to interpret regular tactile interactions with physical objects.

Consider the playing cards example: instead of tapping on digital images of cards, you can now touch and hold physical cards to manipulate a digital system with one convenient sensor.

But you get tactile feedback. Rubbing your fingers together feels very tactile.