We featured the idea, and saw it at other consumer innovation labs a decade ago. Mostly it dealt with using the mirror for cosmetics options and delivering basic 'news and weather' information in the morning. That alone is fairly easy to do. A mostly one way visual and audio interface. Stationary to places in the home, the office, or even in conference rooms. But it seems the demand today is beyond the basic.
A new example is being promoted: " ... The problem with the Perseus smart mirror is the same as with all of the ones that preceded it: over-promising a set of features that will be incredibly difficult to deliver on. These include: voice recognition, video streaming, an HD camera with time-lapse capabilities, fully customizable widgets, third-party apps, and the list goes on .. " . Video at the link.
Some of the idea is similar to the Amazon Echo with a display. The capability is stationary. You could mount any number of them in place. Say as part of wall displays in an office. The inclusion of camera enables conferencing and gestural interaction, as well as displaying tailored information. But that's beyond a mirror.
Also in retail to sell things that we wear like clothes, cosmetics and eyeglasses. Experimented with several of these examples.
See my previous mirror posts.