Re: I'll believe it when I see it
Reminds one of another company, holding a somewhat analogous position during the last years of the last century. I remember Microsoft showing off 'prototypes' that would wow reviewers, even though, just by looking at it, you'd know it simply couldn't be built.
Furthermore, I see this as evidence that Google has now well and truly been taken over by the engineers' hive-mind; even if they could bring something like this to market, and even if it would catch on, the ramifications for society would be staggering. Already people are becoming less social in the real world, but at least they're still forced to interact with it. With devices such as these, the necessity to interact with other people falls rapidly to nill.
As this is a prototype, the features involved are of the mundane Google variety. But imagine having a holographic assistant, telling you where to go, what's going on, helping you find the things (she tells you) you need. Imagine Siri, but with the holographically projected body of a slim, 19yo Chinese woman that knows all your needs and desires, better even than your parents or pet-hamster. How boring would conversation be to a person like that if the other person needs to 'download' the 'conversational parameters' i.e. possess a working knowledge of who you are and what you do, before anything of interest could be addressed.
Remember the old Futurama slogan: "Don't have sex with robots!" True in the future, true now.