I can see a select group of people wanting this.
A person who wants to perform on stage without losing their mobility will benefit from this, but given what you can do with this, it does seem a bit limited. With only 8 triggers on the hand, it may cause a limited set of options. Then again, the video doesn't show if its possible to trigger a new action if you hold 2 or more buttons at the same time.
This type of dj'ing does not give you much to actually play a melody, even if pentatonic, but that may be still possible on 2 octaves for how high your hand is, or perhaps the horizontal position. Panning should not be something the artist wants to use anyway, because with loops its much better to do that at design time.
How does that holding the iphone in your hand works? I see you have some kind of wrist device. Do you get one if you buy this?
Last note, I am buying this product solely because I do not own an iDevice. I have an android phone that I can do everything with, so there's no need to get an iDevice either, and if I did, it'd be an iPad.