I just tried the trial version. I already liked the Nintendo DS game this idea has born and also had the idea to build a clone of it. But I think you did great on it!
For my opinion, I think it would be better if the circles would appear A LITTLE BIT earlier. I only played the two songs on medium - so I don't know whether it exists - but I miss some kind of "hold and follow while holding". Sometimes it's confusing where to press first - but I think this is practice.
Well. I stay with the trial version because I like it, but I wait a little bit, because four/five songs is not that much to play long. But I keep an eye on it