Whenever I see something like this, I immediately think of gorilla arm syndrome. There's a reason I was saying they will never become widespread when all my friends were screaming about Minority Report-style interfaces (ever since Minority Report).
"Gorilla arm" was a term originally tied to touch screens that the user had to reach out and touch a screen in front of them. If the Leap can detect small motions of your hands as they lay on the desk, then using one would look nothing like a "minority report" interface, and avoid gorilla arm.
When typing, you don't really wait for the text to show up on screen before going on to the next letter, and chances are you're hitting backspace before you recognize the letters because you already know you screwed up.
With this interface, you'll have no such luck. You'll type slower, have more errors, and also have muscle strain from not having a more immediate response.