Former tennis player here, and builder of a similar device for weightlifting form. Unlike a lot of other wearable kickstarter projects, I think most of the technological claims here are legit. That said, I'm not sure if it does enough to be useful for an avid tennis player. To improve my game, I would like to see:<p>1) Swing form correction<p>2) The angle of the racket relative to an incoming ball (to predict air or ground balls)<p>3) Stance information.<p>Form correction would require position data during the entire swing. Position information is two integrals away from acceleration data collected from the sensors, so it's far too noisy to determine that well enough.<p>I don't know how would you would measure stance with a bracelet, and I'd imagine that racket angle is unreliable.<p>I also question the ball speed measurement for a similar reason to form correction. I'd imagine a possibly effective way to measure speed would be to correlate acceleration data with externally measured velocity data collected in real-life trials. That said, unless you're competing at an advanced level, speed is a vanity metric so I don't care if its a ballpark estimate.<p>Video recording, however inconvenient it may be, addresses all these well. I'd rather see a camera added to tennis courts to record your performance, but that's far less sexy of a product to build.<p>That said, the bracelet looks sexy and the goal of personal training sure beats the hell out of fitbit/fuel/jawbone's capabilities.