I sent a couple of email questions to the OP and he responded.
The first question I asked him was whether the app calculates an average speed. He said that the current version is only calculating the average speed, which is not good. Assuming that the difference between the initial speed and speed at landing is about 27%, the average speed due to air resistance should be about 16-17% off the initial speed. So, he is risking getting bad reviews due to "inaccurate" serve speed.
The second question I asked him was whether the app is doing 60 fps or 30 fps for my Iphone 4S. He said he programmed the phone to use 60 fps, rather than its default 30 fps rate. This means the resolution should be plus or minus about 5 mph (different between two adjacent frames) at high speed (i.e., over 110 mph).
My conclusion: His use of 60 fps is really great, but until the app calculates the initial serve speed, it's not ready for purchase IMO.
If you want to purchase the app, I think you should use the app for counting the frames and use the chart (see link below) to convert it to the initial speed.