Couple comments on your site, since the app doesn't seem to be available yet.
Your Info page says "more than 500 people up-voted this app at the Tennis Warehouse community." Does this mean that 500 people expressed interest in the app on your poll? If so, your page is misleading, because it implies that the Tennis Warehouse community has actually tried out your app.
Also, you have a tennis ball graphic and a little video graphic on the homepage that lead right back to the homepage. Not so good to have links to nowhere.
Also, I hope you'll eventually reveal the secret algorithm you use for calculating serve speed-- seems a little questionable to ask people to pay money for a tool which relies on calculations that are kept hidden. Just curious, how have you verified that your app is accurate?