Just wanted to say that I like it! I normally track my games on Tennis Match Point by Snazzy Software, but I really like the scoring on the watch. I've been playing competitively for 25 years, so I don't usually forget the score. It happens, though.
TMP is likely a piece of abandonware. You should check it out. Some of what it does could be incorporated into your app. What I like about TMP is it records my scores playing different opponents, and gives me stats on the basics of how I'm performing over time. If yours could track who I'm playing and compile the stats over time, I could see paying $5 for it.
On a side note, your app very much reminds me of a watch I had as a kid that kept score of games and sets. Fun trip down memory lane!