As you can tell, it doesn't sound like a real violin, but an electronic violin. I realize that this could seem fun to some people, and just to try it for technology advancement sake, is interesting to me. Although, I can't see playing any fast songs using this app. The two are not always together and not every note is accurately in tune. Playing with others in a group would be very difficult.
If you'd like to learn the violin, I would suggest spending time learning the real violin and not on this app. You'll have a better sound, more control of the sound, play using techniques that give the violin variety and uniqueness, can play in a group, and play accurate notes.
I think this app is a fun idea to draw more people into music and playing instruments.