The real product (if it ever actually happens) is never going to be as nice as is demonstrated in this demo video.
1) I doubt the visuals will look that nice in real life. Especially in a bright environment. I also wonder how it would look on different skin tones.
2) Every time you place your finger down you would be casting a shadow on the rest of the "display" so it will definitely not look as it is advertised in that video.
3) The battery on that tiny thing would likely drain so quickly that the only thing it would be useful for is as a novelty to show off to your friends for 5 minutes. The battery will drain like crazy even if it is "only" projecting a screen capture of your smartphone casted via WiFi and relaying the touch events back to the smartphone. (And in that case there will probably be annoying lag as well.)
4) Frankly, it would make more sense to just strap on a smartphone to your forearm, assuming you are willing to look like a complete nerd in public.