OMG! Just recently got the iPhone 6 Plus and what a difference it has made in my tennis lessons. The screen is large, not too large; still fits on my hip, and the definition is excellent. The student receives immediate feedback and they love the immediate gratification. The slo-mo (slow motion) function is outstanding! You can stop and pinpoint a problem easily. I know cameras and video players have been around a long time, but they are cumbersome to use.
The kids nowadays are very connected electronically and most players love to see themselves on video. I can send the video to them and or their parents. It is a natural way to aid the learning progress for kids and adults alike. You know the expression, "a picture is worth a thousand words," and this is a perfect description for a method that helps the student's learning curve. I believe the immediate result of the device helps to keep the student's attention and add a dimension of fun!
I wish everyone a Happy and Healthy New Year and may 2015 be filled with exciting matches with great friends.
Good luck on the courts!