I have not read all the comments that others have wrote so i don't bias myself with what most people are thinking. So far i have just watched the video and read the short description of it.
My impressions are this, it is definitely a unique design. I have never seen it anywhere before. However, most people fit their bike once, hopefully perfectly and never mess with it again. Once you fit the stem hight there is really no reason to readjust it. In fact, i would hate my stem high ever changing once i have it perfect.
That being said, i see this product as something i could attach to my bike and ride with it for a little while to find out what my preferred stem hight is - on the fly. Afterwards, i would want to take it off to save weight, less moving parts, less reason to steal my bike. . etc
So the question which begs is, would i buy it? No.
Although the traditional method of determining stem hight is slightly tedious through trial and error, it absolutely works. Would i pay $50+ for the convenience of not having to mess with a allen key? No.
Edit:
I suppose i should clarify my riding style. I am a 1 1/2 hour a day city commuter. I just read in someone else's post that this could be really good for the touring group. I totally agree with that. On such long rides packing so much gear, comfort is a priority over weight. Touring/randonneuring cyclists would be a good target market - i think
Hope this helps!