Wow, most people find that Mugi is one of the hardest? ^^; While I haven't unlocked all of the songs, I managed to get all A rank in hard mode using her. IMO Mio and Azusa are the hardest >_<
At first, I have difficulty in playing the game, but later I found out that as long as you remember the song and the rhythm for the specified instruments, it would be much easier
Also love the customization, although I think they should include more hairstyle variation XD